线程 - 为什么锁定必须遵循try和finally

Question

一个锁总是后跟一个try/finally块,为什么？

ReentrantReadWriteLock readWriteLockBitmap = new ReentrantReadWriteLock();
Lock read = readWriteLockBitmap.readLock();
Lock write = readWriteLockBitmap.writeLock();
int shared = 0;

public void function1(){
    read.lock();
    try{
        //reading shared int
    }
    finally{
        read.unlock();
    }
}

public void function 2(){
    write.lock();
    try{
        //modify shared int
    }
    finally{
        write.unlock();
    }
}

为什么有这个try/finally块而不是简单地编写代码如下:

ReentrantReadWriteLock readWriteLockBitmap = new ReentrantReadWriteLock();
Lock read = readWriteLockBitmap.readLock();
Lock write = readWriteLockBitmap.writeLock();
int shared = 0;

public void function1(){
    read.lock();
    //reading shared int
    read.unlock();
}

public void function 2(){
    write.lock();
    //modify shared int
    write.unlock();
}

Answer 1

因为try/finally块是保证一段代码在另一段完成后执行的唯一方法.

你问为什么不这样做:

public void function1(){
    read.lock();
    this.getSharedInt();
    read.unlock();
}

this.getSharedInt()抛出异常会发生什么？然后你的read.unlock()行将不会执行,导致程序死锁.当然,它可能100%经过认证,现在不会抛出异常,但是当您重构将共享int存储在文件或数据库中时会发生什么？

最后,不要忘记try/finally还会考虑错误,即使函数保证不抛出任何异常,运行时几乎可以在程序中的任何行抛出错误.

请注意,此代码也可以,但它会吞下异常.finally相反,使用允许异常正常传播,同时在所有条件下仍然可以解锁.

public void function2(){
    read.lock();
    try {
        this.getSharedInt();
    } catch(Throwable t) {}
    read.unlock();
}

Answer 2

如果出现任何问题(异常被抛出等),您要确保无论如何都要释放锁.这只是标准做法,即使在这种情况下在技术上可能是不必要的.

Answer 3

这样就不会发生这样的事情：

private static ReentrantReadWriteLock readWriteLockBitmap = new ReentrantReadWriteLock();
private static Lock read = readWriteLockBitmap.readLock();
private static Lock write = readWriteLockBitmap.writeLock();
private static int shared = 0;

public static void function1() {
    read.lock();
    somethingThatMightThrowAnException();
    read.unlock();
}

private static void somethingThatMightThrowAnException() {
    throw new RuntimeException("I'm a bad man.");
}

public static void function2() {
    write.lock();
    //modify shared int
    write.unlock();
}

public static void main(String[] args) {
    try {
        function1();
    } catch (Exception e) {
        System.out.println("Got an exception, but so what?");
    }
    function2();
}