JVM中的執行緒行為

JVM做了它想做的事情，那麼如何預測執行緒執行的順序呢？

獲取原始碼

找到你的第一個執行緒：Java的main()方法

public class MainThread {
 
 public static void main(String... mainThread) {
   System.out.println(Thread.currentThread().getName());
 }
}複製程式碼

Java執行緒生命週期

併發處理：擴充套件Thread類

public class InheritingThread extends Thread {
 
 InheritingThread(String threadName) {
 super(threadName);
 }
 
 public static void main(String... inheriting) {
 System.out.println(Thread.currentThread().getName() + " is running");
 
 new InheritingThread("inheritingThread").start();
 }
 
 @Override
 public void run() {
 System.out.println(Thread.currentThread().getName() + " is running");
 }
}複製程式碼

main is running. inheritingThread is running.

Runnable介面

public class RunnableThread implements Runnable {
 
 public static void main(String... runnableThread) {
 System.out.println(Thread.currentThread().getName());
 
 new Thread(new RunnableThread()).start();
 }
 
 @Override
 public void run() {
 System.out.println(Thread.currentThread().getName());
 }
}複製程式碼

非守護執行緒vs守護執行緒

import java.util.stream.IntStream;
public class NonDaemonAndDaemonThread {
 
 public static void main(String... nonDaemonAndDaemon) throws InterruptedException {
 System.out.println("Starting the execution in the Thread " + Thread.currentThread().getName());
 
 Thread daemonThread = new Thread(() -> IntStream.rangeClosed(1, 100000)
 .forEach(System.out::println));
 
        daemonThread.setDaemon(true);
        daemonThread.start();
 
 Thread.sleep(10);
 
 System.out.println("End of the execution in the Thread " + 
 Thread.currentThread().getName());
 }
}複製程式碼

執行緒優先順序和JVM

 /**
    * The minimum priority that a thread can have.
     */
 public static final int MIN_PRIORITY = 1;
 
 /**
     * The default priority that is assigned to a thread.
     */
 public static final int NORM_PRIORITY = 5;
 
 /**
     * The maximum priority that a thread can have.
     */
 public static final int MAX_PRIORITY = 10;複製程式碼

public class ThreadPriority {
 
 public static void main(String... threadPriority) {
 Thread moeThread = new Thread(() -> System.out.println("Moe"));
 Thread barneyThread = new Thread(() -> System.out.println("Barney"));
 Thread homerThread = new Thread(() -> System.out.println("Homer"));
 
        moeThread.setPriority(Thread.MAX_PRIORITY);
        barneyThread.setPriority(Thread.NORM_PRIORITY);
        homerThread.setPriority(Thread.MIN_PRIORITY);
 
        homerThread.start();
        barneyThread.start();
        moeThread.start();
 }
}複製程式碼

常數與列舉

Java執行緒挑戰！

public class ThreadChallenge {
 private static int wolverineAdrenaline = 10;
 
 public static void main(String... doYourBest) {
 new Motorcycle("Harley Davidson").start();
 
 Motorcycle fastBike = new Motorcycle("Dodge Tomahawk");
        fastBike.setPriority(Thread.MAX_PRIORITY);
        fastBike.setDaemon(false);
        fastBike.start();
 
 Motorcycle yamaha = new Motorcycle("Yamaha YZF");
        yamaha.setPriority(Thread.MIN_PRIORITY);
        yamaha.start();
 }
 
 static class Motorcycle extends Thread {
 Motorcycle(String bikeName) { super(bikeName); }
 
 @Override public void run() {
            wolverineAdrenaline++;
 if (wolverineAdrenaline == 13) {
    System.out.println(this.getName());
 }
 }
 }
}複製程式碼

A. Harley Davidson B. Dodge Tomahawk C. Yamaha YZF D. Indeterminate

剛剛發生了什麼？瞭解執行緒行為

Java執行緒常見錯誤

• · 呼叫該run()方法以嘗試啟動新執行緒。
• · 試圖啟動一個執行緒兩次（這將導致一個IllegalThreadStateException）。
• · 允許多個程式在不應更改狀態時更改物件的狀態。
• · 編寫依賴於執行緒優先順序的程式邏輯（你無法預測它）。
• · 依賴於執行緒執行的順序 - 即使我們首先啟動一個執行緒，也不能保證它將首先被執行。

關於Java執行緒要記住什麼

• · 呼叫start()方法啟動Thread。
• · 可以直接擴充套件Thread類以使用執行緒。
• · 可以在Runnable介面內實現執行緒動作。
• · 執行緒優先順序取決於JVM實現。
• · 執行緒行為將始終取決於JVM實現。
• · 如果非守護程式執行緒首先結束，則守護程式執行緒將無法完成。

Learn more about Java threads on JavaWorld

• · Read the Java 101 threads series to learn more about threads and runnables, thread synchronization, thread scheduling with wait/notify, and thread death.
• · Modern threading: A Java concurrency primer introduces java.util.concurrent and answers common questions for developers new to Java concurrency.
• · Modern threading for not-quite-beginners offers more advanced tips and best practices for working with java.util.concurrent.

More from Rafael

• · Get more quick code tips: Read all the posts in the Java Challengers series.
• · Build your Java skills: Visit the Java Dev Gym for a code workout.
• · Want to work on stress free projects and write bug-free code? Visit the NoBugsProject for your copy of No Bugs, No Stress - Create a Life-Changing Software Without Destroying Your Life.