线程访问同步块/代码Java

问题描述

我正在阅读。以下是示例：

I was reading Synchronized working. Here is the example:

public class Singleton{

private static volatile Singleton _instance;

public static Singleton getInstance(){
   if(_instance == null){
            synchronized(Singleton.class){
              if(_instance == null)
              _instance = new Singleton();
            }
   }
   return _instance;
}

假设两个线程 A 和 B 正在访问 getInstance（）; 方法，
如果线程 在 synchronized 块中，然后线程 B 将跳过该块并执行下一个块/语句或将等待/阻止，直到线程 A 保留 synchronized 块。

Let Suppose two Thread A and B are accessing getInstance(); method,If thread A is in synchronized block then thread B will skip that block and execute next block/statement or will wait/blocked until Thread A leave the synchronized block.

2nd这是什么，为什么 Singleton.class 在 synchronized 参数中以及何时可以 null

2nd What is, why is Singleton.class in synchronized parameter and when it could be null

以及语句是真的吗？

class A
{
   public synchronized void method1(){...}
   public synchronized void method2(){...}
}

如果线程A在 method1 中，那么 threadB 无法输入 method2 或任何其他同步方法。

If thread A is in method1 then threadB cannot enter method2 or any other synchronized method .

推荐答案

1：线程B将等待，直到线程A将释放同步对象上的锁并执行代码，之后它将获取同步对象上的锁。

1: Thread B will wait, until Thread A will release the lock on the synchronized object and execute the code, after it will aquire the lock on the synchronized object.

2： Singleton.class 是表示该类的对象。您正在同步它，因为您的 _instance -object为空。

2: Singleton.class is the object, that represent that class. You are synchronizing on it, since your _instance-object is null.

public synchronized void method1(){...}

正在对象上进行同步，就是你调用那个方法，也就是说，2个线程会互相等待，如果你这样称呼：

is synchronizing on the Object, on that you call that method, that means, 2 Threads will wait for each other, if you call it like this:

final A a = new A();
new Thread(new Runnable(){
    public void run(){
        a.method1();
    }
}).start();
a.method1();

但如果你在不同的对象上调用它们，两个线程将并行执行：

but both threads will be executed parallel, if you call it on different Objects:

A a = new A();
final A b = new A();
new Thread(new Runnable(){
    public void run(){
        b.method1();
    }
}).start();
a.method1();

最后一个问题：对，线程B不会进入方法2，因为同步方法锁定了对象

last question: right, Thread B will not enter method 2, since the synchronized method locks on the Object

Btw。

public synchronized void method1(){...}

相当于：

public void method1(){
    synchronized(this){
        ...
    }
}

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