java - 静态方法内的同步块(synchronized block)将获取类级锁或对象级锁

Question

在下面的代码片段中，我有一个问题。线程将获取类级锁还是对象级锁？

private static Object lock=new Object();
private static Object lock2=new Object();

public static void m1(){
synchronized(lock){
//statements
}
}

public static void m2(){
synchronized(lock2){
//statements
}
}

Answer 1

每个对象都有一个“监视器”。当您使用同步块(synchronized block)时，您可以指定要在其监视器上同步的实例。除了同步 block 之外，还有同步方法。同步实例方法将获取调用该方法的实例的监视器，而同步静态方法将获取java.lang.Class的监视器 封闭类的对象。

public class Foo {

  private static final Object STATIC_LOCK = new Object();
  private final Object instanceLock = new Object();

  public static void bar() {
    synchronized (STATIC_LOCK) { // acquires monitor of "STATIC_LOCK" instance
      // guarded code
    }
  }

  public static synchronized void baz() { // acquires monitor of "Foo.class" instance
    // guarded code
  }

  public void qux() {
    synchronized (instanceLock) { // acquires monitor of "instanceLock" instance
      // guarded code
    }
  }

  public synchronized void quux() { // acquires monitor of "this" instance
    // guarded code
  }
}