Synchronized对实例对象的成员变量同步时是否还会进行代码的重排序

Question

代码如下图所示，代码逻辑主要是启动多个线程，对同一个实例的成员变量进行同步和操作：

public class SynTest {

    private Integer obj = 1;
    /**
     * @param args
     */
    public static void main(String[] args) {
        final SynTest synTest = new SynTest();
        for (int i = 0; i < 10; i++) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    synTest.add();
                }
            }).start();
        }
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
             e.printStackTrace();
        }
    }
    public void add() {
        synchronized (obj) {
            System.out.println(Thread.currentThread());
            obj++;
            System.out.println(Thread.currentThread() + "===" + obj);
            obj++;
            System.out.println(Thread.currentThread() + "-" + obj);
            obj++;
            System.out.println(Thread.currentThread());
        }
    }
}

运行结果如下：

我的问题点主要是：  

1. 为什么单个线程的操作不是连续的？我理解只有一个线程获取到锁，然后进行操作并输出结果，那么输出结果应该是一个线程输出完，另一个线程才会继续输出

2.JVM会对同步的代码块进行同步消除优化？实际的代码应该是和下图所示？

Answer 1

1. 为什么单线程操作不是连续的，这是因为虽然你使用了  synchronized (obj) {}  来尝试获取排它锁，但是其他线程把obj修改了，比如从1修改到2 ，又因为Integer使用了享元模式，1 和 2 不是同一个对象，所以你的每个线程可能锁的不是同一个对象，所以也就无法按照你的需求输出，将obj修改为final常量后，各个线程锁定的肯定是同一个对象了，也就可看到连续输出了。

2. 第二个问题不太熟悉，直接使用 javap -c synTest.class 查看字节码如下,因为 synchronized  进入监视器的字节码指令是 monitorenter ，退出监视器的字节码命令是monitorexit，而这两个命令仅仅出现一次分别是第6行指令和106行指令，所以并没有同步消除优化

public void add();
    Code:
       0: aload_0
       1: getfield      #3                  // Field obj:Ljava/lang/Object;
       4: dup
       5: astore_1
       6: monitorenter
       7: getstatic     #15                 // Field java/lang/System.out:Ljava/io/PrintStream;
      10: invokestatic  #16                 // Method java/lang/Thread.currentThread:()Ljava/lang/Thread;
      13: invokevirtual #17                 // Method java/lang/Thread.getName:()Ljava/lang/String;
      16: invokevirtual #18                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      19: getstatic     #15                 // Field java/lang/System.out:Ljava/io/PrintStream;
      22: new           #19                 // class java/lang/StringBuilder
      25: dup
      26: invokespecial #20                 // Method java/lang/StringBuilder."<init>":()V
      29: invokestatic  #16                 // Method java/lang/Thread.currentThread:()Ljava/lang/Thread;
      32: invokevirtual #17                 // Method java/lang/Thread.getName:()Ljava/lang/String;
      35: invokevirtual #21                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      38: ldc           #22                 // String ===
      40: invokevirtual #21                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      43: aload_0
      44: getfield      #3                  // Field obj:Ljava/lang/Object;
      47: invokevirtual #23                 // Method java/lang/StringBuilder.append:(Ljava/lang/Object;)Ljava/lang/StringBuilder;
      50: invokevirtual #24                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      53: invokevirtual #18                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      56: getstatic     #15                 // Field java/lang/System.out:Ljava/io/PrintStream;
      59: new           #19                 // class java/lang/StringBuilder
      62: dup
      63: invokespecial #20                 // Method java/lang/StringBuilder."<init>":()V
      66: invokestatic  #16                 // Method java/lang/Thread.currentThread:()Ljava/lang/Thread;
      69: invokevirtual #17                 // Method java/lang/Thread.getName:()Ljava/lang/String;
      72: invokevirtual #21                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      75: ldc           #25                 // String -
      77: invokevirtual #21                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      80: aload_0
      81: getfield      #3                  // Field obj:Ljava/lang/Object;
      84: invokevirtual #23                 // Method java/lang/StringBuilder.append:(Ljava/lang/Object;)Ljava/lang/StringBuilder;
      87: invokevirtual #24                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      90: invokevirtual #18                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      93: getstatic     #15                 // Field java/lang/System.out:Ljava/io/PrintStream;
      96: invokestatic  #16                 // Method java/lang/Thread.currentThread:()Ljava/lang/Thread;
      99: invokevirtual #17                 // Method java/lang/Thread.getName:()Ljava/lang/String;
     102: invokevirtual #18                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
     105: aload_1
     106: monitorexit
     107: goto          115
     110: astore_2
     111: aload_1
     112: monitorexit
     113: aload_2

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