如何检查线程是否在同步块或方法中？

Question

My Java application uses two Threads. Historically grown, there are synchronized methods and dedicated lock objects in use. I need to know wether the current thread has a lock and if it is by method or object. How can I do this?

Answer 1

When entering a synchronized method the VM sets a lock on the current object. Thus the following codes have the same effect:

synchronized void syncMethod() {
    // do something
}

void syncManually() {
    synchronized (this) {
        // do something
    }
}

That means the synchronized method does exactly the same as

synchronized( lock ) {
    // do something
}

anywhere in your code.

You can use Thread.holdsLock(...) to check if the thread holds a specific lock. Here is an example code:

final Object lock = new Object(); 

public void lockDemo() {

    System.out.println( Thread.holdsLock(lock) );     // false
    System.out.println( Thread.holdsLock(this) );     // false

    synchronized ( lock ) { 
      System.out.println( Thread.holdsLock(lock) );   // true: locked by object
      System.out.println( Thread.holdsLock(this) );   // false
    }

    doSyncMethod();
}

public synchronized void doSyncMethod() {
    System.out.println( Thread.holdsLock(lock) );  // false
    System.out.println( Thread.holdsLock(this) );  // true: locked by synchronized method
}

Since Java 1.5 more sophisticated locks like ReentrantReadWriteLock are supported by the java.util.concurrent.locks package. They can provide separated read and write locking and improve the performance of your application. The Lock Objects chapter of the Oracle Java Tutorials is a good start here.

Answer 2

If you manually request a thread-dump the JVM will print out a stack-trace of each thread including what objects/methods are locked or waiting on a synchronized block.

You can manually request a thread-dump by sending the JVM process a SIGQUIT in *nix or by typing CTRL-Break (not Ctrl-C ) in a Windows command-prompt window where the JVM was started.