為什麼要寫一個jvm的工具入門呢，一是自己也不會，二是因為不會所以想學一下，還有就是這個確實很重要，對學習jvm有很多的用處，對定位問題有很大的便利，以前畢業那會我記得有個筆試，知道JAVA_HOME/bin目錄的工具嗎，例舉下3個以上，並說說有什麼用，可以怎麼使用，，，，我只能說呵呵了，寫程式碼哪裡要注意這麼多哈
一、基礎瞭解
我們一般安裝java，首先都會來一個java命令（現階段java8和11用的人比較多，我這裡還是8）
D:\Data\lugh1>java 用法: java [-options] class [args...] (執行類) 或 java [-options] -jar jarfile [args...] (執行 jar 檔案) 其中選項包括: -d32 使用 32 位資料模型 (如果可用) -d64 使用 64 位資料模型 (如果可用) -server 選擇 "server" VM 預設 VM 是 server. -cp <目錄和 zip/jar 檔案的類搜尋路徑> -classpath <目錄和 zip/jar 檔案的類搜尋路徑> 用 ; 分隔的目錄, JAR 檔案 和 ZIP 檔案列表, 用於搜尋類檔案。 -D<名稱>=<值> 設定系統屬性 -verbose:[class|gc|jni] 啟用詳細輸出 -version 輸出產品版本並退出 -version:<值> 警告: 此功能已過時, 將在 未來發行版中刪除。 需要指定的版本才能執行 -showversion 輸出產品版本並繼續 -jre-restrict-search | -no-jre-restrict-search 警告: 此功能已過時, 將在 未來發行版中刪除。 在版本搜尋中包括/排除使用者專用 JRE -? -help 輸出此幫助訊息 -X 輸出非標準選項的幫助 -ea[:<packagename>...|:<classname>] -enableassertions[:<packagename>...|:<classname>] 按指定的粒度啟用斷言 -da[:<packagename>...|:<classname>] -disableassertions[:<packagename>...|:<classname>] 禁用具有指定粒度的斷言 -esa | -enablesystemassertions 啟用系統斷言 -dsa | -disablesystemassertions 禁用系統斷言 -agentlib:<libname>[=<選項>] 載入本機代理庫 <libname>, 例如 -agentlib:hprof 另請參閱 -agentlib:jdwp=help 和 -agentlib:hprof=help -agentpath:<pathname>[=<選項>] 按完整路徑名載入本機代理庫 -javaagent:<jarpath>[=<選項>] 載入 Java 程式語言代理, 請參閱 java.lang.instrument -splash:<imagepath> 使用指定的影像顯示啟動螢幕
引數分為三類，分別是：
標準引數（很穩定的）
-help
-version
-X引數 （非標準引數，不能保證被所有的JVM實現都支援）
-Xint
-Xcomp
-XX引數（使用率較高）
-XX:newSize
-XX:+UseSerialGC
反正以前我是不知道-X，-XX，+UseSerialGC中的+號是什麼，或許現在知道，好像也比較容易忘記，因為也沒經常使用呀
來試一下java -X
D:\Data\lugh1>java -X -Xmixed 混合模式執行 (預設) -Xint 僅解釋模式執行 -Xbootclasspath:<用 ; 分隔的目錄和 zip/jar 檔案> 設定搜尋路徑以引導類和資源 -Xbootclasspath/a:<用 ; 分隔的目錄和 zip/jar 檔案> 附加在引導類路徑末尾 -Xbootclasspath/p:<用 ; 分隔的目錄和 zip/jar 檔案> 置於引導類路徑之前 -Xdiag 顯示附加診斷訊息 -Xnoclassgc 禁用類垃圾收集 -Xincgc 啟用增量垃圾收集 -Xloggc:<file> 將 GC 狀態記錄在檔案中 (帶時間戳) -Xbatch 禁用後臺編譯 -Xms<size> 設定初始 Java 堆大小 -Xmx<size> 設定最大 Java 堆大小 -Xss<size> 設定 Java 執行緒堆疊大小 -Xprof 輸出 cpu 配置檔案資料 -Xfuture 啟用最嚴格的檢查, 預期將來的預設值 -Xrs 減少 Java/VM 對作業系統訊號的使用 (請參閱文件) -Xcheck:jni 對 JNI 函式執行其他檢查 -Xshare:off 不嘗試使用共享類資料 -Xshare:auto 在可能的情況下使用共享類資料 (預設) -Xshare:on 要求使用共享類資料, 否則將失敗。 -XshowSettings 顯示所有設定並繼續 -XshowSettings:all 顯示所有設定並繼續 -XshowSettings:vm 顯示所有與 vm 相關的設定並繼續 -XshowSettings:properties 顯示所有屬性設定並繼續 -XshowSettings:locale 顯示所有與區域設定相關的設定並繼續
-Xms<size> 設定初始 Java 堆大小
-Xmx<size> 設定最大 Java 堆大小
-Xss<size> 設定 Java 執行緒堆疊大小
是不是覺得上面幾個引數很熟悉，就是日常調整堆和棧大小的引數呀，很好記吧，比如java ‐Xms2048m ‐Xmx2048m class_name
在解釋模式(interpreted mode)下，-Xint標記會強制JVM執行所有的位元組碼，當然這會降低執行速度，通常低10倍或更多
-Xcomp引數與它（-Xint）正好相反，JVM在第一次使用時會把所有的位元組碼編譯成原生程式碼，從而帶來最大程度的優化。然而，很多應用在使用-Xcomp也會有一些效能損失，當然這比使用-Xint損失的少，原因是-xcomp沒有讓JVM啟用JIT編譯器的全部功能。JIT編譯器可以對是否需要編譯做判斷，如果所有程式碼都進行編譯的話，對於一些只執行一次的程式碼就沒有意義了
-Xmixed是混合模式，將解釋模式與編譯模式進行混合使用，由jvm自己決定，這是jvm預設的模式，也是推薦使用的模式
接下來了解下-XX引數：
-XX引數的使用有2種方式，一種是boolean型別，一種是非boolean型別（好像有點廢話）
- boolean型別
格式：-XX:[+-]
如：-XX:+DisableExplicitGC 表示禁用手動呼叫gc操作，也就是說呼叫System.gc()無效，+表示啟用，-表示禁止吧
非boolean型別
格式：-XX: 如：-XX:NewRatio=1 表示新生代和老年代的比值
那麼多引數怎麼記得呢，可以使用java -XX:+PrintFlagsFinal檢視所有引數呀
D:\MyData\lugh1>java -XX:+PrintFlagsFinal [Global flags] intx ActiveProcessorCount = -1 {product} uintx AdaptiveSizeDecrementScaleFactor = 4 {product} uintx AdaptiveSizeMajorGCDecayTimeScale = 10 {product} uintx AdaptiveSizePausePolicy = 0 {product} uintx AdaptiveSizePolicyCollectionCostMargin = 50 {product} uintx AdaptiveSizePolicyInitializingSteps = 20 {product} uintx AdaptiveSizePolicyOutputInterval = 0 {product} uintx AdaptiveSizePolicyWeight = 10 {product} uintx AdaptiveSizeThroughPutPolicy = 0 {product} uintx AdaptiveTimeWeight = 25 {product} bool AdjustConcurrency = false {product} bool AggressiveHeap = false {product} bool AggressiveOpts = false {product} intx AliasLevel = 3 {C2 product} bool AlignVector = false {C2 product} intx AllocateInstancePrefetchLines = 1 {product} intx AllocatePrefetchDistance = 192 {product} intx AllocatePrefetchInstr = 3 {product} intx AllocatePrefetchLines = 4 {product} intx AllocatePrefetchStepSize = 64 {product} intx AllocatePrefetchStyle = 1 {product} bool AllowJNIEnvProxy = false {product} bool AllowNonVirtualCalls = false {product} bool AllowParallelDefineClass = false {product} bool AllowUserSignalHandlers = false {product} bool AlwaysActAsServerClassMachine = false {product} bool AlwaysCompileLoopMethods = false {product} bool AlwaysLockClassLoader = false {product} bool AlwaysPreTouch = false {product} bool AlwaysRestoreFPU = false {product} bool AlwaysTenure = false {product} bool AssertOnSuspendWaitFailure = false {product} bool AssumeMP = false {product} intx AutoBoxCacheMax = 128 {C2 product} uintx AutoGCSelectPauseMillis = 5000 {product} intx BCEATraceLevel = 0 {product} intx BackEdgeThreshold = 100000 {pd product} bool BackgroundCompilation = true {pd product} uintx BaseFootPrintEstimate = 268435456 {product} intx BiasedLockingBulkRebiasThreshold = 20 {product} intx BiasedLockingBulkRevokeThreshold = 40 {product} intx BiasedLockingDecayTime = 25000 {product} intx BiasedLockingStartupDelay = 4000 {product} bool BindGCTaskThreadsToCPUs = false {product} bool BlockLayoutByFrequency = true {C2 product} intx BlockLayoutMinDiamondPercentage = 20 {C2 product} bool BlockLayoutRotateLoops = true {C2 product} bool BranchOnRegister = false {C2 product} bool BytecodeVerificationLocal = false {product} bool BytecodeVerificationRemote = true {product} bool C1OptimizeVirtualCallProfiling = true {C1 product} bool C1ProfileBranches = true {C1 product} bool C1ProfileCalls = true {C1 product} bool C1ProfileCheckcasts = true {C1 product} bool C1ProfileInlinedCalls = true {C1 product} bool C1ProfileVirtualCalls = true {C1 product} bool C1UpdateMethodData = true {C1 product} intx CICompilerCount := 3 {product} bool CICompilerCountPerCPU = true {product} bool CITime = false {product} bool CMSAbortSemantics = false {product} uintx CMSAbortablePrecleanMinWorkPerIteration = 100 {product} intx CMSAbortablePrecleanWaitMillis = 100 {manageable} uintx CMSBitMapYieldQuantum = 10485760 {product} uintx CMSBootstrapOccupancy = 50 {product} bool CMSClassUnloadingEnabled = true {product} uintx CMSClassUnloadingMaxInterval = 0 {product} bool CMSCleanOnEnter = true {product} bool CMSCompactWhenClearAllSoftRefs = true {product} uintx CMSConcMarkMultiple = 32 {product} bool CMSConcurrentMTEnabled = true {product} uintx CMSCoordinatorYieldSleepCount = 10 {product} bool CMSDumpAtPromotionFailure = false {product} bool CMSEdenChunksRecordAlways = true {product} uintx CMSExpAvgFactor = 50 {product} bool CMSExtrapolateSweep = false {product} uintx CMSFullGCsBeforeCompaction = 0 {product} uintx CMSIncrementalDutyCycle = 10 {product} uintx CMSIncrementalDutyCycleMin = 0 {product} bool CMSIncrementalMode = false {product} uintx CMSIncrementalOffset = 0 {product} bool CMSIncrementalPacing = true {product} uintx CMSIncrementalSafetyFactor = 10 {product} uintx CMSIndexedFreeListReplenish = 4 {product} intx CMSInitiatingOccupancyFraction = -1 {product} uintx CMSIsTooFullPercentage = 98 {product} double CMSLargeCoalSurplusPercent = 0.950000 {product} double CMSLargeSplitSurplusPercent = 1.000000 {product} bool CMSLoopWarn = false {product} uintx CMSMaxAbortablePrecleanLoops = 0 {product} intx CMSMaxAbortablePrecleanTime = 5000 {product} uintx CMSOldPLABMax = 1024 {product} uintx CMSOldPLABMin = 16 {product} uintx CMSOldPLABNumRefills = 4 {product} uintx CMSOldPLABReactivityFactor = 2 {product} bool CMSOldPLABResizeQuicker = false {product} uintx CMSOldPLABToleranceFactor = 4 {product} bool CMSPLABRecordAlways = true {product} uintx CMSParPromoteBlocksToClaim = 16 {product} bool CMSParallelInitialMarkEnabled = true {product} bool CMSParallelRemarkEnabled = true {product} bool CMSParallelSurvivorRemarkEnabled = true {product} uintx CMSPrecleanDenominator = 3 {product} uintx CMSPrecleanIter = 3 {product} uintx CMSPrecleanNumerator = 2 {product} bool CMSPrecleanRefLists1 = true {product} bool CMSPrecleanRefLists2 = false {product} bool CMSPrecleanSurvivors1 = false {product} bool CMSPrecleanSurvivors2 = true {product} uintx CMSPrecleanThreshold = 1000 {product} bool CMSPrecleaningEnabled = true {product} bool CMSPrintChunksInDump = false {product} bool CMSPrintEdenSurvivorChunks = false {product} bool CMSPrintObjectsInDump = false {product} uintx CMSRemarkVerifyVariant = 1 {product} bool CMSReplenishIntermediate = true {product} uintx CMSRescanMultiple = 32 {product} uintx CMSSamplingGrain = 16384 {product} bool CMSScavengeBeforeRemark = false {product} uintx CMSScheduleRemarkEdenPenetration = 50 {product} uintx CMSScheduleRemarkEdenSizeThreshold = 2097152 {product} uintx CMSScheduleRemarkSamplingRatio = 5 {product} double CMSSmallCoalSurplusPercent = 1.050000 {product} double CMSSmallSplitSurplusPercent = 1.100000 {product} bool CMSSplitIndexedFreeListBlocks = true {product} intx CMSTriggerInterval = -1 {manageable} uintx CMSTriggerRatio = 80 {product} intx CMSWaitDuration = 2000 {manageable} uintx CMSWorkQueueDrainThreshold = 10 {product} bool CMSYield = true {product} uintx CMSYieldSleepCount = 0 {product} uintx CMSYoungGenPerWorker = 67108864 {pd product} uintx CMS_FLSPadding = 1 {product} uintx CMS_FLSWeight = 75 {product} uintx CMS_SweepPadding = 1 {product} uintx CMS_SweepTimerThresholdMillis = 10 {product} uintx CMS_SweepWeight = 75 {product} bool CheckEndorsedAndExtDirs = false {product} bool CheckJNICalls = false {product} bool ClassUnloading = true {product} bool ClassUnloadingWithConcurrentMark = true {product} intx ClearFPUAtPark = 0 {product} bool ClipInlining = true {product} uintx CodeCacheExpansionSize = 65536 {pd product} uintx CodeCacheMinimumFreeSpace = 512000 {product} bool CollectGen0First = false {product} bool CompactFields = true {product} intx CompilationPolicyChoice = 3 {product} ccstrlist CompileCommand = {product} ccstr CompileCommandFile = {product} ccstrlist CompileOnly = {product} intx CompileThreshold = 10000 {pd product} bool CompilerThreadHintNoPreempt = true {product} intx CompilerThreadPriority = -1 {product} intx CompilerThreadStackSize = 0 {pd product} uintx CompressedClassSpaceSize = 1073741824 {product} uintx ConcGCThreads = 0 {product} intx ConditionalMoveLimit = 3 {C2 pd product} intx ContendedPaddingWidth = 128 {product} bool ConvertSleepToYield = true {pd product} bool ConvertYieldToSleep = false {product} bool CrashOnOutOfMemoryError = false {product} bool CreateMinidumpOnCrash = false {product} bool CriticalJNINatives = true {product} bool DTraceAllocProbes = false {product} bool DTraceMethodProbes = false {product} bool DTraceMonitorProbes = false {product} bool Debugging = false {product} uintx DefaultMaxRAMFraction = 4 {product} intx DefaultThreadPriority = -1 {product} intx DeferPollingPageLoopCount = -1 {product} intx DeferThrSuspendLoopCount = 4000 {product} bool DeoptimizeRandom = false {product} bool DisableAttachMechanism = false {product} bool DisableExplicitGC = false {product} bool DisplayVMOutputToStderr = false {product} bool DisplayVMOutputToStdout = false {product} bool DoEscapeAnalysis = true {C2 product} bool DontCompileHugeMethods = true {product} bool DontYieldALot = false {pd product} ccstr DumpLoadedClassList = {product} bool DumpReplayDataOnError = true {product} bool DumpSharedSpaces = false {product} bool EagerXrunInit = false {product} intx EliminateAllocationArraySizeLimit = 64 {C2 product} bool EliminateAllocations = true {C2 product} bool EliminateAutoBox = true {C2 product} bool EliminateLocks = true {C2 product} bool EliminateNestedLocks = true {C2 product} intx EmitSync = 0 {product} bool EnableContended = true {product} bool EnableResourceManagementTLABCache = true {product} bool EnableSharedLookupCache = true {product} bool EnableTracing = false {product} uintx ErgoHeapSizeLimit = 0 {product} ccstr ErrorFile = {product} ccstr ErrorReportServer = {product} double EscapeAnalysisTimeout = 20.000000 {C2 product} bool EstimateArgEscape = true {product} bool ExitOnOutOfMemoryError = false {product} bool ExplicitGCInvokesConcurrent = false {product} bool ExplicitGCInvokesConcurrentAndUnloadsClasses = false {product} bool ExtendedDTraceProbes = false {product} ccstr ExtraSharedClassListFile = {product} bool FLSAlwaysCoalesceLarge = false {product} uintx FLSCoalescePolicy = 2 {product} double FLSLargestBlockCoalesceProximity = 0.990000 {product} bool FailOverToOldVerifier = true {product} bool FastTLABRefill = true {product} intx FenceInstruction = 0 {ARCH product} intx FieldsAllocationStyle = 1 {product} bool FilterSpuriousWakeups = true {product} ccstr FlightRecorderOptions = {product} bool ForceNUMA = false {product} bool ForceTimeHighResolution = false {product} intx FreqInlineSize = 325 {pd product} double G1ConcMarkStepDurationMillis = 10.000000 {product} uintx G1ConcRSHotCardLimit = 4 {product} uintx G1ConcRSLogCacheSize = 10 {product} intx G1ConcRefinementGreenZone = 0 {product} intx G1ConcRefinementRedZone = 0 {product} intx G1ConcRefinementServiceIntervalMillis = 300 {product} uintx G1ConcRefinementThreads = 0 {product} intx G1ConcRefinementThresholdStep = 0 {product} intx G1ConcRefinementYellowZone = 0 {product} uintx G1ConfidencePercent = 50 {product} uintx G1HeapRegionSize = 0 {product} uintx G1HeapWastePercent = 5 {product} uintx G1MixedGCCountTarget = 8 {product} intx G1RSetRegionEntries = 0 {product} uintx G1RSetScanBlockSize = 64 {product} intx G1RSetSparseRegionEntries = 0 {product} intx G1RSetUpdatingPauseTimePercent = 10 {product} intx G1RefProcDrainInterval = 10 {product} uintx G1ReservePercent = 10 {product} uintx G1SATBBufferEnqueueingThresholdPercent = 60 {product} intx G1SATBBufferSize = 1024 {product} intx G1UpdateBufferSize = 256 {product} bool G1UseAdaptiveConcRefinement = true {product} uintx GCDrainStackTargetSize = 64 {product} uintx GCHeapFreeLimit = 2 {product} uintx GCLockerEdenExpansionPercent = 5 {product} bool GCLockerInvokesConcurrent = false {product} uintx GCLogFileSize = 8192 {product} uintx GCPauseIntervalMillis = 0 {product} uintx GCTaskTimeStampEntries = 200 {product} uintx GCTimeLimit = 98 {product} uintx GCTimeRatio = 99 {product} uintx HeapBaseMinAddress = 2147483648 {pd product} bool HeapDumpAfterFullGC = false {manageable} bool HeapDumpBeforeFullGC = false {manageable} bool HeapDumpOnOutOfMemoryError = false {manageable} ccstr HeapDumpPath = {manageable} uintx HeapFirstMaximumCompactionCount = 3 {product} uintx HeapMaximumCompactionInterval = 20 {product} uintx HeapSizePerGCThread = 87241520 {product} bool IgnoreEmptyClassPaths = false {product} bool IgnoreUnrecognizedVMOptions = false {product} uintx IncreaseFirstTierCompileThresholdAt = 50 {product} bool IncrementalInline = true {C2 product} uintx InitialBootClassLoaderMetaspaceSize = 4194304 {product} uintx InitialCodeCacheSize = 2555904 {pd product} uintx InitialHeapSize := 132120576 {product} uintx InitialRAMFraction = 64 {product} double InitialRAMPercentage = 1.562500 {product} uintx InitialSurvivorRatio = 8 {product} uintx InitialTenuringThreshold = 7 {product} uintx InitiatingHeapOccupancyPercent = 45 {product} bool Inline = true {product} ccstr InlineDataFile = {product} intx InlineSmallCode = 2000 {pd product} bool InlineSynchronizedMethods = true {C1 product} bool InsertMemBarAfterArraycopy = true {C2 product} intx InteriorEntryAlignment = 16 {C2 pd product} intx InterpreterProfilePercentage = 33 {product} bool JNIDetachReleasesMonitors = true {product} bool JavaMonitorsInStackTrace = true {product} intx JavaPriority10_To_OSPriority = -1 {product} intx JavaPriority1_To_OSPriority = -1 {product} intx JavaPriority2_To_OSPriority = -1 {product} intx JavaPriority3_To_OSPriority = -1 {product} intx JavaPriority4_To_OSPriority = -1 {product} intx JavaPriority5_To_OSPriority = -1 {product} intx JavaPriority6_To_OSPriority = -1 {product} intx JavaPriority7_To_OSPriority = -1 {product} intx JavaPriority8_To_OSPriority = -1 {product} intx JavaPriority9_To_OSPriority = -1 {product} bool LIRFillDelaySlots = false {C1 pd product} uintx LargePageHeapSizeThreshold = 134217728 {product} uintx LargePageSizeInBytes = 0 {product} bool LazyBootClassLoader = true {product} intx LiveNodeCountInliningCutoff = 40000 {C2 product} bool LogCommercialFeatures = false {product} intx LoopMaxUnroll = 16 {C2 product} intx LoopOptsCount = 43 {C2 product} intx LoopUnrollLimit = 60 {C2 pd product} intx LoopUnrollMin = 4 {C2 product} bool LoopUnswitching = true {C2 product} bool ManagementServer = false {product} uintx MarkStackSize = 4194304 {product} uintx MarkStackSizeMax = 536870912 {product} uintx MarkSweepAlwaysCompactCount = 4 {product} uintx MarkSweepDeadRatio = 1 {product} intx MaxBCEAEstimateLevel = 5 {product} intx MaxBCEAEstimateSize = 150 {product} uintx MaxDirectMemorySize = 0 {product} bool MaxFDLimit = true {product} uintx MaxGCMinorPauseMillis = 4294967295 {product} uintx MaxGCPauseMillis = 4294967295 {product} uintx MaxHeapFreeRatio = 100 {manageable} uintx MaxHeapSize := 2111832064 {product} intx MaxInlineLevel = 9 {product} intx MaxInlineSize = 35 {product} intx MaxJNILocalCapacity = 65536 {product} intx MaxJavaStackTraceDepth = 1024 {product} intx MaxJumpTableSize = 65000 {C2 product} intx MaxJumpTableSparseness = 5 {C2 product} intx MaxLabelRootDepth = 1100 {C2 product} intx MaxLoopPad = 11 {C2 product} uintx MaxMetaspaceExpansion = 5451776 {product} uintx MaxMetaspaceFreeRatio = 70 {product} uintx MaxMetaspaceSize = 4294901760 {product} uintx MaxNewSize := 703594496 {product} intx MaxNodeLimit = 75000 {C2 product} uint64_t MaxRAM = 0 {pd product} uintx MaxRAMFraction = 4 {product} double MaxRAMPercentage = 25.000000 {product} intx MaxRecursiveInlineLevel = 1 {product} uintx MaxTenuringThreshold = 15 {product} intx MaxTrivialSize = 6 {product} intx MaxVectorSize = 32 {C2 product} uintx MetaspaceSize = 21807104 {pd product} bool MethodFlushing = true {product} uintx MinHeapDeltaBytes := 524288 {product} uintx MinHeapFreeRatio = 0 {manageable} intx MinInliningThreshold = 250 {product} intx MinJumpTableSize = 10 {C2 pd product} uintx MinMetaspaceExpansion = 339968 {product} uintx MinMetaspaceFreeRatio = 40 {product} uintx MinRAMFraction = 2 {product} double MinRAMPercentage = 50.000000 {product} uintx MinSurvivorRatio = 3 {product} uintx MinTLABSize = 2048 {product} intx MonitorBound = 0 {product} bool MonitorInUseLists = false {product} intx MultiArrayExpandLimit = 6 {C2 product} bool MustCallLoadClassInternal = false {product} uintx NUMAChunkResizeWeight = 20 {product} uintx NUMAInterleaveGranularity = 2097152 {product} uintx NUMAPageScanRate = 256 {product} uintx NUMASpaceResizeRate = 1073741824 {product} bool NUMAStats = false {product} ccstr NativeMemoryTracking = off {product} bool NeedsDeoptSuspend = false {pd product} bool NeverActAsServerClassMachine = false {pd product} bool NeverTenure = false {product} uintx NewRatio = 2 {product} uintx NewSize := 44040192 {product} uintx NewSizeThreadIncrease = 5320 {pd product} intx NmethodSweepActivity = 10 {product} intx NmethodSweepCheckInterval = 5 {product} intx NmethodSweepFraction = 16 {product} intx NodeLimitFudgeFactor = 2000 {C2 product} uintx NumberOfGCLogFiles = 0 {product} intx NumberOfLoopInstrToAlign = 4 {C2 product} intx ObjectAlignmentInBytes = 8 {lp64_product} uintx OldPLABSize = 1024 {product} uintx OldPLABWeight = 50 {product} uintx OldSize := 88080384 {product} bool OmitStackTraceInFastThrow = true {product} ccstrlist OnError = {product} ccstrlist OnOutOfMemoryError = {product} intx OnStackReplacePercentage = 140 {pd product} bool OptimizeFill = true {C2 product} bool OptimizePtrCompare = true {C2 product} bool OptimizeStringConcat = true {C2 product} bool OptoBundling = false {C2 pd product} intx OptoLoopAlignment = 16 {pd product} bool OptoScheduling = false {C2 pd product} uintx PLABWeight = 75 {product} bool PSChunkLargeArrays = true {product} intx ParGCArrayScanChunk = 50 {product} uintx ParGCDesiredObjsFromOverflowList = 20 {product} bool ParGCTrimOverflow = true {product} bool ParGCUseLocalOverflow = false {product} uintx ParallelGCBufferWastePct = 10 {product} uintx ParallelGCThreads = 4 {product} bool ParallelGCVerbose = false {product} uintx ParallelOldDeadWoodLimiterMean = 50 {product} uintx ParallelOldDeadWoodLimiterStdDev = 80 {product} bool ParallelRefProcBalancingEnabled = true {product} bool ParallelRefProcEnabled = false {product} bool PartialPeelAtUnsignedTests = true {C2 product} bool PartialPeelLoop = true {C2 product} intx PartialPeelNewPhiDelta = 0 {C2 product} uintx PausePadding = 1 {product} intx PerBytecodeRecompilationCutoff = 200 {product} intx PerBytecodeTrapLimit = 4 {product} intx PerMethodRecompilationCutoff = 400 {product} intx PerMethodTrapLimit = 100 {product} bool PerfAllowAtExitRegistration = false {product} bool PerfBypassFileSystemCheck = false {product} intx PerfDataMemorySize = 32768 {product} intx PerfDataSamplingInterval = 50 {product} ccstr PerfDataSaveFile = {product} bool PerfDataSaveToFile = false {product} bool PerfDisableSharedMem = false {product} intx PerfMaxStringConstLength = 1024 {product} intx PreInflateSpin = 10 {pd product} bool PreferInterpreterNativeStubs = false {pd product} intx PrefetchCopyIntervalInBytes = 576 {product} intx PrefetchFieldsAhead = 1 {product} intx PrefetchScanIntervalInBytes = 576 {product} bool PreserveAllAnnotations = false {product} bool PreserveFramePointer = false {pd product} uintx PretenureSizeThreshold = 0 {product} bool PrintAdaptiveSizePolicy = false {product} bool PrintCMSInitiationStatistics = false {product} intx PrintCMSStatistics = 0 {product} bool PrintClassHistogram = false {manageable} bool PrintClassHistogramAfterFullGC = false {manageable} bool PrintClassHistogramBeforeFullGC = false {manageable} bool PrintCodeCache = false {product} bool PrintCodeCacheOnCompilation = false {product} bool PrintCommandLineFlags = false {product} bool PrintCompilation = false {product} bool PrintConcurrentLocks = false {manageable} intx PrintFLSCensus = 0 {product} intx PrintFLSStatistics = 0 {product} bool PrintFlagsFinal := true {product} bool PrintFlagsInitial = false {product} bool PrintGC = false {manageable} bool PrintGCApplicationConcurrentTime = false {product} bool PrintGCApplicationStoppedTime = false {product} bool PrintGCCause = true {product} bool PrintGCDateStamps = false {manageable} bool PrintGCDetails = false {manageable} bool PrintGCID = false {manageable} bool PrintGCTaskTimeStamps = false {product} bool PrintGCTimeStamps = false {manageable} bool PrintHeapAtGC = false {product rw} bool PrintHeapAtGCExtended = false {product rw} bool PrintHeapAtSIGBREAK = true {product} bool PrintJNIGCStalls = false {product} bool PrintJNIResolving = false {product} bool PrintOldPLAB = false {product} bool PrintOopAddress = false {product} bool PrintPLAB = false {product} bool PrintParallelOldGCPhaseTimes = false {product} bool PrintPromotionFailure = false {product} bool PrintReferenceGC = false {product} bool PrintSafepointStatistics = false {product} intx PrintSafepointStatisticsCount = 300 {product} intx PrintSafepointStatisticsTimeout = -1 {product} bool PrintSharedArchiveAndExit = false {product} bool PrintSharedDictionary = false {product} bool PrintSharedSpaces = false {product} bool PrintStringDeduplicationStatistics = false {product} bool PrintStringTableStatistics = false {product} bool PrintTLAB = false {product} bool PrintTenuringDistribution = false {product} bool PrintTieredEvents = false {product} bool PrintVMOptions = false {product} bool PrintVMQWaitTime = false {product} bool PrintWarnings = true {product} uintx ProcessDistributionStride = 4 {product} bool ProfileInterpreter = true {pd product} bool ProfileIntervals = false {product} intx ProfileIntervalsTicks = 100 {product} intx ProfileMaturityPercentage = 20 {product} bool ProfileVM = false {product} bool ProfilerPrintByteCodeStatistics = false {product} bool ProfilerRecordPC = false {product} uintx PromotedPadding = 3 {product} uintx QueuedAllocationWarningCount = 0 {product} uintx RTMRetryCount = 5 {ARCH product} bool RangeCheckElimination = true {product} intx ReadPrefetchInstr = 0 {ARCH product} bool ReassociateInvariants = true {C2 product} bool ReduceBulkZeroing = true {C2 product} bool ReduceFieldZeroing = true {C2 product} bool ReduceInitialCardMarks = true {C2 product} bool ReduceSignalUsage = false {product} intx RefDiscoveryPolicy = 0 {product} bool ReflectionWrapResolutionErrors = true {product} bool RegisterFinalizersAtInit = true {product} bool RelaxAccessControlCheck = false {product} ccstr ReplayDataFile = {product} bool RequireSharedSpaces = false {product} uintx ReservedCodeCacheSize = 251658240 {pd product} bool ResizeOldPLAB = true {product} bool ResizePLAB = true {product} bool ResizeTLAB = true {pd product} bool RestoreMXCSROnJNICalls = false {product} bool RestrictContended = true {product} bool RewriteBytecodes = true {pd product} bool RewriteFrequentPairs = true {pd product} intx SafepointPollOffset = 256 {C1 pd product} intx SafepointSpinBeforeYield = 2000 {product} bool SafepointTimeout = false {product} intx SafepointTimeoutDelay = 10000 {product} bool ScavengeBeforeFullGC = true {product} intx SelfDestructTimer = 0 {product} uintx SharedBaseAddress = 0 {product} ccstr SharedClassListFile = {product} uintx SharedMiscCodeSize = 122880 {product} uintx SharedMiscDataSize = 4194304 {product} uintx SharedReadOnlySize = 16777216 {product} uintx SharedReadWriteSize = 16777216 {product} bool ShowMessageBoxOnError = false {product} intx SoftRefLRUPolicyMSPerMB = 1000 {product} bool SpecialEncodeISOArray = true {C2 product} bool SplitIfBlocks = true {C2 product} intx StackRedPages = 1 {pd product} intx StackShadowPages = 6 {pd product} bool StackTraceInThrowable = true {product} intx StackYellowPages = 3 {pd product} bool StartAttachListener = false {product} intx StarvationMonitorInterval = 200 {product} bool StressLdcRewrite = false {product} uintx StringDeduplicationAgeThreshold = 3 {product} uintx StringTableSize = 60013 {product} bool SuppressFatalErrorMessage = false {product} uintx SurvivorPadding = 3 {product} uintx SurvivorRatio = 8 {product} intx SuspendRetryCount = 50 {product} intx SuspendRetryDelay = 5 {product} intx SyncFlags = 0 {product} ccstr SyncKnobs = {product} intx SyncVerbose = 0 {product} uintx TLABAllocationWeight = 35 {product} uintx TLABRefillWasteFraction = 64 {product} uintx TLABSize = 0 {product} bool TLABStats = true {product} uintx TLABWasteIncrement = 4 {product} uintx TLABWasteTargetPercent = 1 {product} uintx TargetPLABWastePct = 10 {product} uintx TargetSurvivorRatio = 50 {product} uintx TenuredGenerationSizeIncrement = 20 {product} uintx TenuredGenerationSizeSupplement = 80 {product} uintx TenuredGenerationSizeSupplementDecay = 2 {product} intx ThreadPriorityPolicy = 0 {product} bool ThreadPriorityVerbose = false {product} uintx ThreadSafetyMargin = 52428800 {product} intx ThreadStackSize = 0 {pd product} uintx ThresholdTolerance = 10 {product} intx Tier0BackedgeNotifyFreqLog = 10 {product} intx Tier0InvokeNotifyFreqLog = 7 {product} intx Tier0ProfilingStartPercentage = 200 {product} intx Tier23InlineeNotifyFreqLog = 20 {product} intx Tier2BackEdgeThreshold = 0 {product} intx Tier2BackedgeNotifyFreqLog = 14 {product} intx Tier2CompileThreshold = 0 {product} intx Tier2InvokeNotifyFreqLog = 11 {product} intx Tier3BackEdgeThreshold = 60000 {product} intx Tier3BackedgeNotifyFreqLog = 13 {product} intx Tier3CompileThreshold = 2000 {product} intx Tier3DelayOff = 2 {product} intx Tier3DelayOn = 5 {product} intx Tier3InvocationThreshold = 200 {product} intx Tier3InvokeNotifyFreqLog = 10 {product} intx Tier3LoadFeedback = 5 {product} intx Tier3MinInvocationThreshold = 100 {product} intx Tier4BackEdgeThreshold = 40000 {product} intx Tier4CompileThreshold = 15000 {product} intx Tier4InvocationThreshold = 5000 {product} intx Tier4LoadFeedback = 3 {product} intx Tier4MinInvocationThreshold = 600 {product} bool TieredCompilation = true {pd product} intx TieredCompileTaskTimeout = 50 {product} intx TieredRateUpdateMaxTime = 25 {product} intx TieredRateUpdateMinTime = 1 {product} intx TieredStopAtLevel = 4 {product} bool TimeLinearScan = false {C1 product} bool TraceBiasedLocking = false {product} bool TraceClassLoading = false {product rw} bool TraceClassLoadingPreorder = false {product} bool TraceClassPaths = false {product} bool TraceClassResolution = false {product} bool TraceClassUnloading = false {product rw} bool TraceDynamicGCThreads = false {product} bool TraceGen0Time = false {product} bool TraceGen1Time = false {product} ccstr TraceJVMTI = {product} bool TraceLoaderConstraints = false {product rw} bool TraceMetadataHumongousAllocation = false {product} bool TraceMonitorInflation = false {product} bool TraceParallelOldGCTasks = false {product} intx TraceRedefineClasses = 0 {product} bool TraceSafepointCleanupTime = false {product} bool TraceSharedLookupCache = false {product} bool TraceSuspendWaitFailures = false {product} intx TrackedInitializationLimit = 50 {C2 product} bool TransmitErrorReport = false {product} bool TrapBasedNullChecks = false {pd product} bool TrapBasedRangeChecks = false {C2 pd product} intx TypeProfileArgsLimit = 2 {product} uintx TypeProfileLevel = 111 {pd product} intx TypeProfileMajorReceiverPercent = 90 {C2 product} intx TypeProfileParmsLimit = 2 {product} intx TypeProfileWidth = 2 {product} intx UnguardOnExecutionViolation = 0 {product} bool UnlinkSymbolsALot = false {product} bool Use486InstrsOnly = false {ARCH product} bool UseAES = true {product} bool UseAESIntrinsics = true {product} intx UseAVX = 2 {ARCH product} bool UseAdaptiveGCBoundary = false {product} bool UseAdaptiveGenerationSizePolicyAtMajorCollection = true {product} bool UseAdaptiveGenerationSizePolicyAtMinorCollection = true {product} bool UseAdaptiveNUMAChunkSizing = true {product} bool UseAdaptiveSizeDecayMajorGCCost = true {product} bool UseAdaptiveSizePolicy = true {product} bool UseAdaptiveSizePolicyFootprintGoal = true {product} bool UseAdaptiveSizePolicyWithSystemGC = false {product} bool UseAddressNop = true {ARCH product} bool UseAltSigs = false {product} bool UseAutoGCSelectPolicy = false {product} bool UseBMI1Instructions = true {ARCH product} bool UseBMI2Instructions = true {ARCH product} bool UseBiasedLocking = true {product} bool UseBimorphicInlining = true {C2 product} bool UseBoundThreads = true {product} bool UseCLMUL = true {ARCH product} bool UseCMSBestFit = true {product} bool UseCMSCollectionPassing = true {product} bool UseCMSCompactAtFullCollection = true {product} bool UseCMSInitiatingOccupancyOnly = false {product} bool UseCRC32Intrinsics = true {product} bool UseCodeCacheFlushing = true {product} bool UseCompiler = true {product} bool UseCompilerSafepoints = true {product} bool UseCompressedClassPointers := true {lp64_product} bool UseCompressedOops := true {lp64_product} bool UseConcMarkSweepGC = false {product} bool UseCondCardMark = false {C2 product} bool UseCountLeadingZerosInstruction = true {ARCH product} bool UseCountTrailingZerosInstruction = true {ARCH product} bool UseCountedLoopSafepoints = false {C2 product} bool UseCounterDecay = true {product} bool UseDivMod = true {C2 product} bool UseDynamicNumberOfGCThreads = false {product} bool UseFPUForSpilling = true {C2 product} bool UseFastAccessorMethods = false {product} bool UseFastEmptyMethods = false {product} bool UseFastJNIAccessors = true {product} bool UseFastStosb = true {ARCH product} bool UseG1GC = false {product} bool UseGCLogFileRotation = false {product} bool UseGCOverheadLimit = true {product} bool UseGCTaskAffinity = false {product} bool UseHeavyMonitors = false {product} bool UseInlineCaches = true {product} bool UseInterpreter = true {product} bool UseJumpTables = true {C2 product} bool UseLWPSynchronization = true {product} bool UseLargePages = false {pd product} bool UseLargePagesInMetaspace = false {product} bool UseLargePagesIndividualAllocation := false {pd product} bool UseLockedTracing = false {product} bool UseLoopCounter = true {product} bool UseLoopInvariantCodeMotion = true {C1 product} bool UseLoopPredicate = true {C2 product} bool UseMathExactIntrinsics = true {C2 product} bool UseMaximumCompactionOnSystemGC = true {product} bool UseMembar = false {pd product} bool UseMontgomeryMultiplyIntrinsic = true {C2 product} bool UseMontgomerySquareIntrinsic = true {C2 product} bool UseMulAddIntrinsic = true {C2 product} bool UseMultiplyToLenIntrinsic = true {C2 product} bool UseNUMA = false {product} bool UseNUMAInterleaving = false {product} bool UseNewLongLShift = false {ARCH product} bool UseOSErrorReporting = false {pd product} bool UseOldInlining = true {C2 product} bool UseOnStackReplacement = true {pd product} bool UseOnlyInlinedBimorphic = true {C2 product} bool UseOptoBiasInlining = true {C2 product} bool UsePSAdaptiveSurvivorSizePolicy = true {product} bool UseParNewGC = false {product} bool UseParallelGC := true {product} bool UseParallelOldGC = true {product} bool UsePerfData = true {product} bool UsePopCountInstruction = true {product} bool UseRDPCForConstantTableBase = false {C2 product} bool UseRTMDeopt = false {ARCH product} bool UseRTMLocking = false {ARCH product} bool UseSHA = false {product} bool UseSHA1Intrinsics = false {product} bool UseSHA256Intrinsics = false {product} bool UseSHA512Intrinsics = false {product} intx UseSSE = 4 {product} bool UseSSE42Intrinsics = true {product} bool UseSerialGC = false {product} bool UseSharedSpaces = false {product} bool UseSignalChaining = true {product} bool UseSquareToLenIntrinsic = true {C2 product} bool UseStoreImmI16 = false {ARCH product} bool UseStringDeduplication = false {product} bool UseSuperWord = true {C2 product} bool UseTLAB = true {pd product} bool UseThreadPriorities = true {pd product} bool UseTypeProfile = true {product} bool UseTypeSpeculation = true {C2 product} bool UseUTCFileTimestamp = true {product} bool UseUnalignedLoadStores = true {ARCH product} bool UseVMInterruptibleIO = false {product} bool UseXMMForArrayCopy = true {product} bool UseXmmI2D = false {ARCH product} bool UseXmmI2F = false {ARCH product} bool UseXmmLoadAndClearUpper = true {ARCH product} bool UseXmmRegToRegMoveAll = true {ARCH product} bool VMThreadHintNoPreempt = false {product} intx VMThreadPriority = -1 {product} intx VMThreadStackSize = 0 {pd product} intx ValueMapInitialSize = 11 {C1 product} intx ValueMapMaxLoopSize = 8 {C1 product} intx ValueSearchLimit = 1000 {C2 product} bool VerifyMergedCPBytecodes = true {product} bool VerifySharedSpaces = false {product} intx WorkAroundNPTLTimedWaitHang = 1 {product} uintx YoungGenerationSizeIncrement = 20 {product} uintx YoungGenerationSizeSupplement = 80 {product} uintx YoungGenerationSizeSupplementDecay = 8 {product} uintx YoungPLABSize = 4096 {product} bool ZeroTLAB = false {product} intx hashCode = 5 {product} 用法: java [-options] class [args...] (執行類) 或 java [-options] -jar jarfile [args...] (執行 jar 檔案) 其中選項包括: -d32 使用 32 位資料模型 (如果可用) -d64 使用 64 位資料模型 (如果可用) -server 選擇 "server" VM 預設 VM 是 server. -cp <目錄和 zip/jar 檔案的類搜尋路徑> -classpath <目錄和 zip/jar 檔案的類搜尋路徑> 用 ; 分隔的目錄, JAR 檔案 和 ZIP 檔案列表, 用於搜尋類檔案。 -D<名稱>=<值> 設定系統屬性 -verbose:[class|gc|jni] 啟用詳細輸出 -version 輸出產品版本並退出 -version:<值> 警告: 此功能已過時, 將在 未來發行版中刪除。 需要指定的版本才能執行 -showversion 輸出產品版本並繼續 -jre-restrict-search | -no-jre-restrict-search 警告: 此功能已過時, 將在 未來發行版中刪除。 在版本搜尋中包括/排除使用者專用 JRE -? -help 輸出此幫助訊息 -X 輸出非標準選項的幫助 -ea[:<packagename>...|:<classname>] -enableassertions[:<packagename>...|:<classname>] 按指定的粒度啟用斷言 -da[:<packagename>...|:<classname>] -disableassertions[:<packagename>...|:<classname>] 禁用具有指定粒度的斷言 -esa | -enablesystemassertions 啟用系統斷言 -dsa | -disablesystemassertions 禁用系統斷言 -agentlib:<libname>[=<選項>] 載入本機代理庫 <libname>, 例如 -agentlib:hprof 另請參閱 -agentlib:jdwp=help 和 -agentlib:hprof=help -agentpath:<pathname>[=<選項>] 按完整路徑名載入本機代理庫 -javaagent:<jarpath>[=<選項>] 載入 Java 程式語言代理, 請參閱 java.lang.instrument -splash:<imagepath> 使用指定的影像顯示啟動螢幕 有關詳細資訊, 請參閱 http://www.oracle.com/technetwork/java/javase/documentation/index.html。
上面的引數中有等號（=和:=），分別代表的是預設值，和被修改的值啦
好了，基礎夠基礎了吧，以後這些加（+）減（-）等於（=或:=）看的懂了
二、基礎工具
$JAVA_HOME/bin目錄下真的有很多的工具，這裡不截圖啦，直接上用的比較的工具好啦
2.1、jps
這個工具真的很實用，而且很簡單呢，功能linux的ps命令類似：可以列出正在執行的虛擬機器程式，並顯示虛擬機器執行主類（Main Class，main()函式所在的類）名稱以及這些程式的本地虛擬機器唯一ID（LVMID，Local Virtual Machine Identifier）
[root@hadoop apache-tomcat-8.5.57]# jps -help usage: jps [-help] jps [-q] [-mlvV] [<hostid>] Definitions: <hostid>: <hostname>[:<port>]
-l : 輸出主類全名或jar路徑 -q : 只輸出LVMID -m : 輸出JVM啟動時傳遞給main()的引數 -v : 輸出JVM啟動時顯示指定的JVM引數
2.2、jstat
jstat（JVM Statistics Monitoring Tool）是用於監視虛擬機器各種執行狀態資訊的命令列工具。它可以顯示本地或者遠端[插圖]虛擬機器程式中的類載入、記憶體、垃圾收集、即時編譯等執行時資料。
[root@hadoop apache-tomcat-8.5.57]# jstat -help Usage: jstat -help|-options jstat -<option> [-t] [-h<lines>] <vmid> [<interval> [<count>]] Definitions: <option> An option reported by the -options option <vmid> Virtual Machine Identifier. A vmid takes the following form: <lvmid>[@<hostname>[:<port>]] Where <lvmid> is the local vm identifier for the target Java virtual machine, typically a process id; <hostname> is the name of the host running the target Java virtual machine; and <port> is the port number for the rmiregistry on the target host. See the jvmstat documentation for a more complete description of the Virtual Machine Identifier. <lines> Number of samples between header lines. <interval> Sampling interval. The following forms are allowed: <n>["ms"|"s"] Where <n> is an integer and the suffix specifies the units as milliseconds("ms") or seconds("s"). The default units are "ms". <count> Number of samples to take before terminating. -J<flag> Pass <flag> directly to the runtime system.
jstat -options #檢視下支援的選項，如下
[root@hadoop apache-tomcat-8.5.57]# jstat -options -class #類載入器 -compiler #JIT -gc #GC堆狀態 -gccapacity #各個區的大小 -gccause #最近一次GC的統計和原因 -gcmetacapacity #元空間大小 -gcnew #新區統計 -gcnewcapacity #新區大小 -gcold #老區統計 -gcoldcapacity #老區大小 -gcutil #GC彙總統計 -printcompilation #HotSpot編譯統計
Options #選項，我們一般使用 -gcutil 檢視gc情況
vmid #VM的程式號，即當前執行的java程式號，可以理解我java程式的pid
interval #間隔時間，單位為秒或者毫秒
count #列印次數，如果預設則列印無數次
對於命令格式中的VMID與LVMID需要特別說明一下：如果是本地虛擬機器程式，VMID與LVMID是一致的；如果是遠端虛擬機器程式，那VMID的格式應當是
[protocol:][//]lvmid[@hostname][:port]/servername]
開始操作：
jstat -class 1645 #類載入的統計
Loaded：載入class的數量
Bytes：所佔用空間大小
Unloaded：未載入數量
Bytes：未載入佔用空間
Time：時間
jstat -gc 1645 1 3 #統計3次，每隔一秒，對gc進行統計
S0C：第一個Survivor區的大小（KB）
S1C：第二個Survivor區的大小（KB）
S0U：第一個Survivor區的使用大小（KB）
S1U：第二個Survivor區的使用大小（KB）
EC：Eden區的大小（KB）
EU：Eden區的使用大小（KB）
OC：Old區大小（KB）
OU：Old使用大小（KB）
MC：方法區大小（KB），java8方法區的實現是後設資料空間，java7的方法區實現是永久代
MU：方法區使用大小（KB）
CCSC：壓縮類空間大小（KB）
CCSU：壓縮類空間使用大小（KB）
YGC：年輕代垃圾回收次數
YGCT：年輕代垃圾回收消耗時間
FGC：老年代垃圾回收次數
FGCT：老年代垃圾回收消耗時間
GCT：垃圾回收消耗總時間
jstat -compiler 1645 #對編譯進行統計
Compiled：編譯數量。
Failed：失敗數量
Invalid：不可用數量
Time：時間
FailedType：失敗型別
FailedMethod：失敗的方法
2.3、jinfo
jinfo（Configuration Info for Java）的作用是實時檢視和調整虛擬機器各項引數
[root@hadoop ~]# jinfo -help Usage: jinfo [option] <pid> (to connect to running process) jinfo [option] <executable <core> (to connect to a core file) jinfo [option] [server_id@]<remote server IP or hostname> (to connect to remote debug server) where <option> is one of: -flag <name> to print the value of the named VM flag #列印單個jvm的引數 -flag [+|-]<name> to enable or disable the named VM flag #執行期修改一部分執行期可寫的虛擬機器引數值 -flag <name>=<value> to set the named VM flag to the given value #執行期修改一部分執行期可寫的虛擬機器引數值 -flags to print VM flags #列印該程式的相關的引數 -sysprops to print Java system properties #把虛擬機器程式的System.getProperties()的內容列印出來 <no option> to print both of the above -h | -help to print this help message
jinfo -flags 1645
jinfo -flag InitialHeapSize 1645
jinfo -flag InitialHeapSize 1645
[root@hadoop ~]# jinfo -sysprops 1645 Attaching to process ID 1645, please wait... Debugger attached successfully. Server compiler detected. JVM version is 25.181-b13 java.runtime.name = Java(TM) SE Runtime Environment java.vm.version = 25.181-b13 sun.boot.library.path = /home/hadoop/jdk/jre/lib/amd64 java.protocol.handler.pkgs = org.apache.catalina.webresources shared.loader = java.vendor.url = http://java.oracle.com/ java.vm.vendor = Oracle Corporation path.separator = : file.encoding.pkg = sun.io java.vm.name = Java HotSpot(TM) 64-Bit Server VM java.util.logging.config.file = /root/apache-tomcat-8.5.57/conf/logging.properties tomcat.util.buf.StringCache.byte.enabled = true sun.os.patch.level = unknown sun.java.launcher = SUN_STANDARD user.country = US tomcat.util.scan.StandardJarScanFilter.jarsToScan = log4j-taglib*.jar,log4j-web*.jar,log4javascript*.jar,slf4j-taglib*.jar user.dir = /root/apache-tomcat-8.5.57 java.vm.specification.name = Java Virtual Machine Specification java.runtime.version = 1.8.0_181-b13 java.awt.graphicsenv = sun.awt.X11GraphicsEnvironment os.arch = amd64 java.endorsed.dirs = /home/hadoop/jdk/jre/lib/endorsed line.separator = java.io.tmpdir = /root/apache-tomcat-8.5.57/temp java.vm.specification.vendor = Oracle Corporation java.util.logging.manager = org.apache.juli.ClassLoaderLogManager java.naming.factory.url.pkgs = org.apache.naming os.name = Linux org.apache.catalina.security.SecurityListener.UMASK = 0027 sun.jnu.encoding = UTF-8 ignore.endorsed.dirs = java.library.path = /usr/java/packages/lib/amd64:/usr/lib64:/lib64:/lib:/usr/lib java.class.version = 52.0 java.specification.name = Java Platform API Specification sun.management.compiler = HotSpot 64-Bit Tiered Compilers os.version = 2.6.32-754.24.3.el6.x86_64 java.util.concurrent.ForkJoinPool.common.threadFactory = org.apache.catalina.startup.SafeForkJoinWorkerThreadFactory user.home = /root user.timezone = Asia/Chongqing catalina.useNaming = true java.awt.printerjob = sun.print.PSPrinterJob file.encoding = UTF-8 java.specification.version = 1.8 tomcat.util.scan.StandardJarScanFilter.jarsToSkip = annotations-api.jar,ant-junit*.jar,ant-launcher.jar,ant.jar,asm-*.jar,aspectj*.jar,bootstrap.jar,catalina-ant.jar,catalina-ha.jar,catalina-jmx-remote.jar,catalina-storeconfig.jar,catalina-tribes.jar,catalina-ws.jar,catalina.jar,cglib-*.jar,cobertura-*.jar,commons-beanutils*.jar,commons-codec*.jar,commons-collections*.jar,commons-daemon.jar,commons-dbcp*.jar,commons-digester*.jar,commons-fileupload*.jar,commons-httpclient*.jar,commons-io*.jar,commons-lang*.jar,commons-logging*.jar,commons-math*.jar,commons-pool*.jar,dom4j-*.jar,easymock-*.jar,ecj-*.jar,el-api.jar,geronimo-spec-jaxrpc*.jar,h2*.jar,hamcrest-*.jar,hibernate*.jar,httpclient*.jar,icu4j-*.jar,jasper-el.jar,jasper.jar,jaspic-api.jar,jaxb-*.jar,jaxen-*.jar,jdom-*.jar,jetty-*.jar,jmx-tools.jar,jmx.jar,jsp-api.jar,jstl.jar,jta*.jar,junit-*.jar,junit.jar,log4j*.jar,mail*.jar,objenesis-*.jar,oraclepki.jar,oro-*.jar,servlet-api-*.jar,servlet-api.jar,slf4j*.jar,taglibs-standard-spec-*.jar,tagsoup-*.jar,tomcat-api.jar,tomcat-coyote.jar,tomcat-dbcp.jar,tomcat-i18n-*.jar,tomcat-jdbc.jar,tomcat-jni.jar,tomcat-juli-adapters.jar,tomcat-juli.jar,tomcat-util-scan.jar,tomcat-util.jar,tomcat-websocket.jar,tools.jar,websocket-api.jar,wsdl4j*.jar,xercesImpl.jar,xml-apis.jar,xmlParserAPIs-*.jar,xmlParserAPIs.jar,xom-*.jar catalina.home = /root/apache-tomcat-8.5.57 user.name = root java.class.path = /root/apache-tomcat-8.5.57/bin/bootstrap.jar:/root/apache-tomcat-8.5.57/bin/tomcat-juli.jar java.naming.factory.initial = org.apache.naming.java.javaURLContextFactory package.definition = sun.,java.,org.apache.catalina.,org.apache.coyote.,org.apache.jasper.,org.apache.naming.,org.apache.tomcat. java.vm.specification.version = 1.8 sun.arch.data.model = 64 sun.java.command = org.apache.catalina.startup.Bootstrap start java.home = /home/hadoop/jdk/jre user.language = en java.specification.vendor = Oracle Corporation awt.toolkit = sun.awt.X11.XToolkit java.vm.info = mixed mode java.version = 1.8.0_181 java.ext.dirs = /home/hadoop/jdk/jre/lib/ext:/usr/java/packages/lib/ext sun.boot.class.path = /home/hadoop/jdk/jre/lib/resources.jar:/home/hadoop/jdk/jre/lib/rt.jar:/home/hadoop/jdk/jre/lib/sunrsasign.jar:/home/hadoop/jdk/jre/lib/jsse.jar:/home/hadoop/jdk/jre/lib/jce.jar:/home/hadoop/jdk/jre/lib/charsets.jar:/home/hadoop/jdk/jre/lib/jfr.jar:/home/hadoop/jdk/jre/classes server.loader = java.vendor = Oracle Corporation catalina.base = /root/apache-tomcat-8.5.57 jdk.tls.ephemeralDHKeySize = 2048 file.separator = / java.vendor.url.bug = http://bugreport.sun.com/bugreport/ common.loader = "${catalina.base}/lib","${catalina.base}/lib/*.jar","${catalina.home}/lib","${catalina.home}/lib/*.jar" sun.io.unicode.encoding = UnicodeLittle sun.cpu.endian = little package.access = sun.,org.apache.catalina.,org.apache.coyote.,org.apache.jasper.,org.apache.tomcat. sun.cpu.isalist =
2.4、jmap
jmap（Memory Map for Java）命令用於生成堆轉儲快照（一般稱為heapdump或dump檔案），還可以使用“暴力”方式：-XX：+HeapDumpOnOutOfMemoryError引數，可以讓虛擬機器在記憶體溢位異常出現之後自動生成堆轉儲快照檔案
jmap的作用並不僅僅是為了獲取堆轉儲快照，它還可以查詢finalize執行佇列、Java堆和方法區的詳細資訊，如空間使用率、當前用的是哪種收集器等
[root@hadoop ~]# jmap -help Usage: jmap [option] <pid> (to connect to running process) jmap [option] <executable <core> (to connect to a core file) jmap [option] [server_id@]<remote server IP or hostname> (to connect to remote debug server) where <option> is one of: <none> to print same info as Solaris pmap -heap to print java heap summary #統計堆 -histo[:live] to print histogram of java object heap; if the "live" suboption is specified, only count live objects -clstats to print class loader statistics -finalizerinfo to print information on objects awaiting finalization -dump:<dump-options> to dump java heap in hprof binary format #dump出java堆的二進位制檔案 dump-options: live dump only live objects; if not specified, all objects in the heap are dumped. format=b binary format file=<file> dump heap to <file> Example: jmap -dump:live,format=b,file=heap.bin <pid> -F force. Use with -dump:<dump-options> <pid> or -histo to force a heap dump or histogram when <pid> does not respond. The "live" suboption is not supported in this mode. -h | -help to print this help message -J<flag> to pass <flag> directly to the runtime system
-heap #列印jvm heap的情況，比如引數，分代，回收器等 -histo #列印jvm heap的直方圖。其輸出資訊包括類名，物件數量，物件佔用大小。 -histo:live #同上，但是隻答應存活物件的情況 -dump #生成堆轉儲快照,格式為-dump:[live,]format=b,file=<filename>,其中自引數說明是否只dump出存活物件 -finalizerinfo #顯示在F-Queue中等待的finalizer執行緒執行finalize方法的物件 -clstats #列印類的載入資訊 -F #強制生成dump檔案
jmap -heap 1645
jmap -histo 1645 | more #檢視記憶體中的物件和數量
上圖中的物件說明：
B byte C char D double F float I int J long Z boolean [ 陣列，如[I表示int[] [L+類名 其他物件
jmap -dump:format=b,file=/root/dump.bin 1645 #將記憶體的使用情況dump到檔案中
[root@hadoop bin]# jmap -dump:format=b,file=/root/dump.bin 1645 Dumping heap to /root/dump.bin ... Heap dump file created
2.5、jhat
JDK提供jhat（JVM Heap Analysis Tool）命令與jmap搭配使用，來分析jmap生成的堆轉儲快照。jhat內建了一個微型的HTTP/Web伺服器，生成堆轉儲快照的分析結果後，可以在瀏覽器中檢視（實際情況使用的不多）
[root@hadoop ~]# jhat -help Usage: jhat [-stack <bool>] [-refs <bool>] [-port <port>] [-baseline <file>] [-debug <int>] [-version] [-h|-help] <file> -J<flag> Pass <flag> directly to the runtime system. For example, -J-mx512m to use a maximum heap size of 512MB -stack false: Turn off tracking object allocation call stack. -refs false: Turn off tracking of references to objects -port <port>: Set the port for the HTTP server. Defaults to 7000 -exclude <file>: Specify a file that lists data members that should be excluded from the reachableFrom query. -baseline <file>: Specify a baseline object dump. Objects in both heap dumps with the same ID and same class will be marked as not being "new". -debug <int>: Set debug level. 0: No debug output 1: Debug hprof file parsing 2: Debug hprof file parsing, no server -version Report version number -h|-help Print this help and exit <file> The file to read For a dump file that contains multiple heap dumps, you may specify which dump in the file by appending "#<number>" to the file name, i.e. "foo.hprof#3". All boolean options default to "true"
jhat -port 9999 dump.bin
登入自己的地址：http://192.168.88.137:9999/
這個網址拉到最下面有一個查詢功能：
如上的每一個結果都可以點進去去檢視
2.6、jstack
jstack（Stack Trace for Java）命令用於生成虛擬機器當前時刻的執行緒快照（一般稱為threaddump或者javacore檔案）。執行緒快照就是當前虛擬機器內每一條執行緒正在執行的方法堆疊的集合，生成執行緒快照的目的通常是定位執行緒出現長時間停頓的原因，如執行緒間死鎖、死迴圈、請求外部資源導致的長時間掛起等，都是導致執行緒長時間停頓的常見原因
[root@hadoop ~]# jstack -help Usage: jstack [-l] <pid> (to connect to running process) jstack -F [-m] [-l] <pid> (to connect to a hung process) jstack [-m] [-l] <executable> <core> (to connect to a core file) jstack [-m] [-l] [server_id@]<remote server IP or hostname> (to connect to a remote debug server) Options: -F to force a thread dump. Use when jstack <pid> does not respond (process is hung) -m to print both java and native frames (mixed mode) -l long listing. Prints additional information about locks -h or -help to print this help message
-F : 當正常輸出請求不被響應時，強制輸出執行緒堆疊 -l : 除堆疊外，顯示關於鎖的附加資訊 -m : 如果呼叫到本地方法的話，可以顯示C/C++的堆疊
jstack 1645 | more #檢視該執行緒的快照資訊
如上的狀態資訊可以參考：java多執行緒與併發（基礎篇）
三、視覺化工具
3.1、JConsole
JConsole（Java Monitoring and Management Console）是一款基於JMX（Java Manage-ment Extensions）的視覺化監視、管理工具。它的主要功能是通過JMX的MBean（Managed Bean）對系統進行資訊收集和引數動態調整
一、本地連線
可以看出來可以對記憶體，執行緒等進行監控。
二、遠端監控
這裡我們使用tomcat做實驗，首先要修改catalina.sh檔案，新增如下內容：參考https://blog.csdn.net/shamoshuizu/article/details/70598268
JAVA_OPTS="-Djava.rmi.server.hostname=192.168.88.137 -Dcom.sun.management.jmxremote.port=8099 -Dcom.sun.management.jmxremote.rmi.port=8099 -Dcom.sun.management.jmxremote.ssl=false -Dcom.sun.management.jmxremote.authenticate=false"
然後重啟tomcat，接下來連線遠端
這個工具使用起來並不難，只要自己要有jvm的基礎啦，不然看不懂，展示出來了也是白乾活。
3.2、VisualVM
VisualVM（All-in-One Java Troubleshooting Tool）是功能最強大的執行監視和故障處理程式之一，能夠監控執行緒，記憶體情況，檢視方法的CPU時間和記憶體中的物件，已被GC的物件，反向檢視分配的堆疊，使用起來也很簡單
雙擊jvisualvm.exe軟體，開啟該工具
同JConsole 一樣可以本地和遠端，直接開啟本地程式
基本pid，jvm引數，主機等一覽無餘，還有監控頁面包括cpu，堆，類，執行緒等監控，還可以執行垃圾回收，和dump執行緒呢
試試遠端的，同JConsole 的設定方式對tomcate進行設定
其實工具是什麼呢，就是為了方便獲取資訊嘛，能不能看懂，能不能還得看自己的水平夠不夠，能不能夠發現問題，能不能解決問題了。
3.3、JMC
Java Mission Control：可持續線上的監控工具
JMC與虛擬機器之間同樣採取JMX協議進行通訊，JMC一方面作為JMX控制檯，顯示來自虛擬機器MBean提供的資料；另一方面作為JFR的分析工具，展示來自JFR的資料
用起來也很簡單，雙擊jmc.exe開啟工具，預設情況下會掃描出本地的程式，然後可以點選：檔案->連線，來建立新的連線：
然後根據下圖操作，遇到情況下一步，下一步就好，就有如下的圖了：
該工具持續監控了很多東西：
一般資訊：關於虛擬機器、作業系統和記錄的一般資訊。
記憶體：關於記憶體管理和垃圾收集的資訊。
程式碼：關於方法、異常錯誤、編譯和類載入的資訊。
執行緒：關於應用程式中執行緒和鎖的資訊。
I/O：關於檔案和套接字輸入、輸出的資訊。
系統：關於正在執行Java虛擬機器的系統、程式和環境變數的資訊。
事件：關於記錄中的事件型別的資訊，可以根據執行緒或堆疊跟蹤，按照日誌或圖形的格式檢視。
四、其他工具-MAT
MAT(Memory Analyzer Tool)，一個基於Eclipse的記憶體分析工具，是一個快速、功能豐富的JAVA heap分析工具，它可以幫助我們查詢記憶體洩漏和減少記憶體消耗。使用記憶體分析工具從眾多的物件中進行分析，快速的計算出在記憶體中物件的佔用大小，看看是誰阻止了垃圾收集器的回收工作，並可以通過報表直觀的檢視到可能造成這種結果的物件
官網：https://www.eclipse.org/mat/
下載地址：https://www.eclipse.org/mat/downloads.php
下載後，解壓，雙擊MemoryAnalyzer.exe開啟程式，然後點選檔案->開啟dump出的檔案，就出現瞭如下的圖
下圖可以看到基本的記憶體可能出問題的地方：
這篇文章純粹是一個工具的使用簡單說明，沒有太多的技術含量，但是我覺得工欲善其事必先利其器，所以用好一些工具對自己能力的提升，效率的提升有莫大的幫助，工具這些東西只有用到的時候才會查詢，但是也要大概瞭解下怎麼用吧，比如什麼工具可以用來幹嘛呢，我寫這個的目的就是用來以後自己當做工具文件來檢視下，技術用時方恨少，所以平時多積累些也不是一件壞事呢，好吧，好好努力撒。