The premise

1. How is visibility implemented for Lock and Synchronzied?

Lock: using the volatile happens-before related rules, Java AbstractQueuedSynchronizer internal hold a volatile modified state variables:

  /** * The synchronization state. * /
    private volatile int state;
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According to the volatile visibility rule, multiple threads always get the latest value of state when they read it. That is, the current thread’s locking and unlocking of a variable is visible to the next thread.

Synchronzied synchronized happens-before the lock is subsequently locked. Of course, you can also see how this works from a bytecode perspective:

public class Demo_01 {
    private final Object lock = new Object();

    public void test(a) {
        synchronized (lock) {
            // do sth....}}}Copy the code

Execute javap -verbose Demo_01 to intercept partial bytecode:

 Code:
      stack=2, locals=3, args_size=1
         0: aload_0
         1: getfield      #3                  // Field lock:Ljava/lang/Object;
         4: dup
         5: astore_1
         6: monitorenter
         7: aload_1
         8: monitorexit
         9: goto          17
        12: astore_2
        13: aload_1
        14: monitorexit
        15: aload_2
        16: athrow
        17: return

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Look at lines 6 and 14 monitorenter and Monitorexit. The code that executes between these two instructions is: sequential, visible, atomic;

In a word: are the implementation of pipe procedures in concurrent programming;

Here are the differences from a programmer’s perspective:

Tip:

  • In an interview, they ask:
What does the keyword synchronized mean when referring to instance methods, static methods, and code blocks?
  • A:

  1. When modifying an instance method, lock the current object instance.

  2. When modifying a static method, lock the current class.

  3. When modifying a code block, locks the specified object.

  4. The lock of the current object/current class/specified object is required before entering the synchronized code/code block.