java中synchronized（同步代码块和同步方法）详解及区别

//下列两个方法有什么区别
public synchronized void method1(){}

public void method2(){
 synchronized (obj){}
}

public class SynObj {
  public synchronized void methodA() {
    System.out.println("methodA.....");
    try {
      Thread.sleep(5000);
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
  }

  public void methodB() {
    synchronized(this) {
      System.out.pritntln("methodB.....");
    }
  }

  public void methodC() {
    String str = "sss";
    synchronized (str) {
      System.out.println(    "methodC.....");
    }
  }
}

public class TestSyn {
  public static void main(String[] args) {
    final SynObj obj = new SynObj();

    Thread t1 = new Thread(new Runnable() {
      @Override
      public void run() {
        obj.methodA();
      }
    });
    t1.start();

    Thread t2 = new Thread(new Runnable() {
      @Override
      public void run() {
        obj.methodB();
      }
    });
    t2.start();

    Thread t3 = new Thread(new Runnable() {
      @Override
      public void run() {
        obj.methodC();
      }
    });
    t3.start();
  }
}

methodA.....
methodC.....
//methodB会隔一段时间才会打印出来
methodB.....