Java同步方法锁定对象或方法? [英] Java synchronized method lock on object, or method?

问题描述

如果我在同一个类中有 2 个同步方法，但每个方法访问不同的变量，2 个线程可以同时访问这 2 个方法吗?锁定发生在对象上，还是像同步方法中的变量一样具体?

If I have 2 synchronized methods in the same class, but each accessing different variables, can 2 threads access those 2 methods at the same time? Does the lock occur on the object, or does it get as specific as the variables inside the synchronized method?

示例:

class X {

    private int a;
    private int b;

    public synchronized void addA(){
        a++;
    }

    public synchronized void addB(){
        b++;
    }

}

2 个线程可以访问同一个类 X 的实例，同时执行 x.addA() 和 x.addB() 吗?

Can 2 threads access the same instance of class X performing x.addA() and x.addB() at the same time?

推荐答案

如果您将方法声明为 synchronized(就像您通过键入 public synchronized void addA()code>) 你在 整个 对象上同步，所以两个线程从同一个对象访问不同的变量无论如何都会相互阻塞.

If you declare the method as synchronized (as you're doing by typing public synchronized void addA()) you synchronize on the whole object, so two thread accessing a different variable from this same object would block each other anyway.

如果你想一次只同步一个变量，这样两个线程在访问不同的变量时不会互相阻塞，你可以在 synchronized() 块中分别对它们进行同步.如果 a 和 b 是对象引用，您将使用:

If you want to synchronize only on one variable at a time, so two threads won't block each other while accessing different variables, you have synchronize on them separately in synchronized () blocks. If a and b were object references you would use:

public void addA() {
    synchronized( a ) {
        a++;
    }
}

public void addB() {
    synchronized( b ) {
        b++;
    }
}

但由于它们是原始类型，您不能这样做.

But since they're primitives you can't do this.

我建议你改用AtomicInteger:

import java.util.concurrent.atomic.AtomicInteger;

class X {

    AtomicInteger a;
    AtomicInteger b;

    public void addA(){
        a.incrementAndGet();
    }

    public void addB(){ 
        b.incrementAndGet();
    }
}

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