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本文主要介紹一下pg jdbc statement的queryTimeout及resultSet的next方法
例項程式
@Test
public void testReadTimeout() throws SQLException {
Connection connection = dataSource.getConnection();
//https://jdbc.postgresql.org/documentation/head/query.html
connection.setAutoCommit(false); //NOTE 為了設定fetchSize,必須設定為false
String sql = "select * from demo_table";
PreparedStatement pstmt;
try {
pstmt = (PreparedStatement)connection.prepareStatement(sql);
pstmt.setQueryTimeout(1); //NOTE 設定Statement執行完成的超時時間,前提是socket的timeout比這個大
pstmt.setFetchSize(5000); //NOTE 這樣設定為了模擬query timeout的異常
System.out.println("ps.getQueryTimeout():" + pstmt.getQueryTimeout());
System.out.println("ps.getFetchSize():" + pstmt.getFetchSize());
System.out.println("ps.getFetchDirection():" + pstmt.getFetchDirection());
System.out.println("ps.getMaxFieldSize():" + pstmt.getMaxFieldSize());
ResultSet rs = pstmt.executeQuery(); //NOTE 設定Statement執行完成的超時時間,前提是socket的timeout比這個大
//NOTE 這裡返回了就代表statement執行完成,預設返回fetchSize的資料
int col = rs.getMetaData().getColumnCount();
System.out.println("============================");
while (rs.next()) { //NOTE 這個的timeout由socket的超時時間設定,oracle.jdbc.ReadTimeout=60000
for (int i = 1; i <= col; i++) {
System.out.print(rs.getObject(i));
}
System.out.println("");
}
System.out.println("============================");
} catch (SQLException e) {
e.printStackTrace();
} finally {
//close resources
}
}
複製程式碼PgStatement
ostgresql-9.4.1212.jre7-sources.jar!/org/postgresql/jdbc/PgStatement.java
executeInternal()
private void executeInternal(CachedQuery cachedQuery, ParameterList queryParameters, int flags)
throws SQLException {
closeForNextExecution();
// Enable cursor-based resultset if possible.
if (fetchSize > 0 && !wantsScrollableResultSet() && !connection.getAutoCommit()
&& !wantsHoldableResultSet()) {
flags |= QueryExecutor.QUERY_FORWARD_CURSOR;
}
if (wantsGeneratedKeysOnce || wantsGeneratedKeysAlways) {
flags |= QueryExecutor.QUERY_BOTH_ROWS_AND_STATUS;
// If the no results flag is set (from executeUpdate)
// clear it so we get the generated keys results.
//
if ((flags & QueryExecutor.QUERY_NO_RESULTS) != 0) {
flags &= ~(QueryExecutor.QUERY_NO_RESULTS);
}
}
if (isOneShotQuery(cachedQuery)) {
flags |= QueryExecutor.QUERY_ONESHOT;
}
// Only use named statements after we hit the threshold. Note that only
// named statements can be transferred in binary format.
if (connection.getAutoCommit()) {
flags |= QueryExecutor.QUERY_SUPPRESS_BEGIN;
}
// updateable result sets do not yet support binary updates
if (concurrency != ResultSet.CONCUR_READ_ONLY) {
flags |= QueryExecutor.QUERY_NO_BINARY_TRANSFER;
}
Query queryToExecute = cachedQuery.query;
if (queryToExecute.isEmpty()) {
flags |= QueryExecutor.QUERY_SUPPRESS_BEGIN;
}
if (!queryToExecute.isStatementDescribed() && forceBinaryTransfers
&& (flags & QueryExecutor.QUERY_EXECUTE_AS_SIMPLE) == 0) {
// Simple `Q` execution does not need to know parameter types
// When binaryTransfer is forced, then we need to know resulting parameter and column types,
// thus sending a describe request.
int flags2 = flags | QueryExecutor.QUERY_DESCRIBE_ONLY;
StatementResultHandler handler2 = new StatementResultHandler();
connection.getQueryExecutor().execute(queryToExecute, queryParameters, handler2, 0, 0,
flags2);
ResultWrapper result2 = handler2.getResults();
if (result2 != null) {
result2.getResultSet().close();
}
}
StatementResultHandler handler = new StatementResultHandler();
result = null;
try {
startTimer();
connection.getQueryExecutor().execute(queryToExecute, queryParameters, handler, maxrows,
fetchSize, flags);
} finally {
killTimerTask();
}
result = firstUnclosedResult = handler.getResults();
if (wantsGeneratedKeysOnce || wantsGeneratedKeysAlways) {
generatedKeys = result;
result = result.getNext();
if (wantsGeneratedKeysOnce) {
wantsGeneratedKeysOnce = false;
}
}
}
複製程式碼注意，這裡在執行前後分別呼叫了startTimer()和killTimerTask()
startTimer()
private void startTimer() {
/*
* there shouldn`t be any previous timer active, but better safe than sorry.
*/
cleanupTimer();
STATE_UPDATER.set(this, StatementCancelState.IN_QUERY);
if (timeout == 0) {
return;
}
TimerTask cancelTask = new TimerTask() {
public void run() {
try {
if (!CANCEL_TIMER_UPDATER.compareAndSet(PgStatement.this, this, null)) {
// Nothing to do here, statement has already finished and cleared
// cancelTimerTask reference
return;
}
PgStatement.this.cancel();
} catch (SQLException e) {
}
}
};
CANCEL_TIMER_UPDATER.set(this, cancelTask);
connection.addTimerTask(cancelTask, timeout);
}
複製程式碼- startTimer呼叫了cleanupTimer()
- cancelTask呼叫的是PgStatement.this.cancel()
- 最後呼叫connection.addTimerTask新增定時任務
cleanupTimer()
/**
* Clears {@link #cancelTimerTask} if any. Returns true if and only if "cancel" timer task would
* never invoke {@link #cancel()}.
*/
private boolean cleanupTimer() {
TimerTask timerTask = CANCEL_TIMER_UPDATER.get(this);
if (timerTask == null) {
// If timeout is zero, then timer task did not exist, so we safely report "all clear"
return timeout == 0;
}
if (!CANCEL_TIMER_UPDATER.compareAndSet(this, timerTask, null)) {
// Failed to update reference -> timer has just fired, so we must wait for the query state to
// become "cancelling".
return false;
}
timerTask.cancel();
connection.purgeTimerTasks();
// All clear
return true;
}
複製程式碼注意這裡更新statement狀態之後，呼叫task的cancel，以及connection.purgeTimerTasks()
cancel()
public void cancel() throws SQLException {
if (!STATE_UPDATER.compareAndSet(this, StatementCancelState.IN_QUERY, StatementCancelState.CANCELING)) {
// Not in query, there`s nothing to cancel
return;
}
try {
// Synchronize on connection to avoid spinning in killTimerTask
synchronized (connection) {
connection.cancelQuery();
}
} finally {
STATE_UPDATER.set(this, StatementCancelState.CANCELLED);
synchronized (connection) {
connection.notifyAll(); // wake-up killTimerTask
}
}
}
複製程式碼executeQuery超時了則直接呼叫connection.cancelQuery()
public void cancelQuery() throws SQLException {
checkClosed();
queryExecutor.sendQueryCancel();
}
複製程式碼postgresql-9.4.1212.jre7-sources.jar!/org/postgresql/core/QueryExecutorBase.java
public void sendQueryCancel() throws SQLException {
if (cancelPid <= 0) {
return;
}
PGStream cancelStream = null;
// Now we need to construct and send a cancel packet
try {
if (logger.logDebug()) {
logger.debug(" FE=> CancelRequest(pid=" + cancelPid + ",ckey=" + cancelKey + ")");
}
cancelStream =
new PGStream(pgStream.getSocketFactory(), pgStream.getHostSpec(), cancelSignalTimeout);
if (cancelSignalTimeout > 0) {
cancelStream.getSocket().setSoTimeout(cancelSignalTimeout);
}
cancelStream.sendInteger4(16);
cancelStream.sendInteger2(1234);
cancelStream.sendInteger2(5678);
cancelStream.sendInteger4(cancelPid);
cancelStream.sendInteger4(cancelKey);
cancelStream.flush();
cancelStream.receiveEOF();
} catch (IOException e) {
// Safe to ignore.
if (logger.logDebug()) {
logger.debug("Ignoring exception on cancel request:", e);
}
} finally {
if (cancelStream != null) {
try {
cancelStream.close();
} catch (IOException e) {
// Ignored.
}
}
}
}
複製程式碼向資料庫server傳送cancel指令
killTimerTask()
private void killTimerTask() {
boolean timerTaskIsClear = cleanupTimer();
// The order is important here: in case we need to wait for the cancel task, the state must be
// kept StatementCancelState.IN_QUERY, so cancelTask would be able to cancel the query.
// It is believed that this case is very rare, so "additional cancel and wait below" would not
// harm it.
if (timerTaskIsClear && STATE_UPDATER.compareAndSet(this, StatementCancelState.IN_QUERY, StatementCancelState.IDLE)) {
return;
}
// Being here means someone managed to call .cancel() and our connection did not receive
// "timeout error"
// We wait till state becomes "cancelled"
boolean interrupted = false;
while (!STATE_UPDATER.compareAndSet(this, StatementCancelState.CANCELLED, StatementCancelState.IDLE)) {
synchronized (connection) {
try {
// Note: wait timeout here is irrelevant since synchronized(connection) would block until
// .cancel finishes
connection.wait(10);
} catch (InterruptedException e) { // NOSONAR
// Either re-interrupt this method or rethrow the "InterruptedException"
interrupted = true;
}
}
}
if (interrupted) {
Thread.currentThread().interrupt();
}
}
複製程式碼這裡先呼叫cleanupTimer,然後更新statement的狀態
PgConnection
postgresql-9.4.1212.jre7-sources.jar!/org/postgresql/jdbc/PgConnection.java
getTimer()
private synchronized Timer getTimer() {
if (cancelTimer == null) {
cancelTimer = Driver.getSharedTimer().getTimer();
}
return cancelTimer;
}
複製程式碼這裡建立或獲取一個timer
addTimerTask()
public void addTimerTask(TimerTask timerTask, long milliSeconds) {
Timer timer = getTimer();
timer.schedule(timerTask, milliSeconds);
}
複製程式碼這個新增timerTask就是直接排程了
purgeTimerTasks()
postgresql-9.4.1212.jre7-sources.jar!/org/postgresql/jdbc/PgConnection.java
public void purgeTimerTasks() {
Timer timer = cancelTimer;
if (timer != null) {
timer.purge();
}
}
複製程式碼在cleanupTimer中被呼叫，用來清理已經被cancel掉的timer task
PgResultSet
postgresql-9.4.1212.jre7-sources.jar!/org/postgresql/jdbc/PgResultSet.java
next()
public boolean next() throws SQLException {
checkClosed();
if (onInsertRow) {
throw new PSQLException(GT.tr("Can``t use relative move methods while on the insert row."),
PSQLState.INVALID_CURSOR_STATE);
}
if (current_row + 1 >= rows.size()) {
if (cursor == null || (maxRows > 0 && row_offset + rows.size() >= maxRows)) {
current_row = rows.size();
this_row = null;
rowBuffer = null;
return false; // End of the resultset.
}
// Ask for some more data.
row_offset += rows.size(); // We are discarding some data.
int fetchRows = fetchSize;
if (maxRows != 0) {
if (fetchRows == 0 || row_offset + fetchRows > maxRows) {
// Fetch would exceed maxRows, limit it.
fetchRows = maxRows - row_offset;
}
}
// Execute the fetch and update this resultset.
connection.getQueryExecutor().fetch(cursor, new CursorResultHandler(), fetchRows);
current_row = 0;
// Test the new rows array.
if (rows.isEmpty()) {
this_row = null;
rowBuffer = null;
return false;
}
} else {
current_row++;
}
initRowBuffer();
return true;
}
複製程式碼這裡的fetch沒有像executeQuery那樣加timer
postgresql-9.4.1212.jre7-sources.jar!/org/postgresql/core/v3/QueryExecutorImpl.java
public synchronized void fetch(ResultCursor cursor, ResultHandler handler, int fetchSize)
throws SQLException {
waitOnLock();
final Portal portal = (Portal) cursor;
// Insert a ResultHandler that turns bare command statuses into empty datasets
// (if the fetch returns no rows, we see just a CommandStatus..)
final ResultHandler delegateHandler = handler;
handler = new ResultHandlerDelegate(delegateHandler) {
public void handleCommandStatus(String status, int updateCount, long insertOID) {
handleResultRows(portal.getQuery(), null, new ArrayList<byte[][]>(), null);
}
};
// Now actually run it.
try {
processDeadParsedQueries();
processDeadPortals();
sendExecute(portal.getQuery(), portal, fetchSize);
sendSync();
processResults(handler, 0);
estimatedReceiveBufferBytes = 0;
} catch (IOException e) {
abort();
handler.handleError(
new PSQLException(GT.tr("An I/O error occurred while sending to the backend."),
PSQLState.CONNECTION_FAILURE, e));
}
handler.handleCompletion();
}
複製程式碼小結
- queryTimeout是採用新增timer來控制，如果請求過多，可能會造成timer過多
timeout時間不宜過長,不過正常執行完sql,會呼叫killTimerTask()方,裡頭會先cleanupTimer,取消timerTask,然後呼叫purgeTimerTasks()清理cancel掉的task,避免timeout時間過長導致task堆積最後記憶體溢位
- 超時之後會timer task會向資料庫server傳送cancel query指令
- 傳送完cancel query指令之後，client端的查詢按預期應該丟擲SQLException(
這裡頭的機制有待深入研究，可能是server端返回timeout error) - executeQuery方法預設會拉取fetchSize的資料並返回
- next()方法根據需要再去fetch，這個fetch方法就沒有timer來限制時間了，但是最底層應該是受socketTimeout限制