java - 线程安全对象数组

Question

我在 Java 中使用二维数组。但是现在，我想将这个类用于多线程。我怎样才能做到这一点？

我知道如何实现线程安全功能(添加同步关键字)。如果同时触发 clear 和 getItem 函数会怎样？对于这种情况，我该如何做线程安全事件？

public class ThreadSafeArray {
    int ROW_MAX_COUNT = 1024;

    int rowCount = 0;
    int counterForRow = 0;
    private Object [][] objInstances = new Object[ROW_MAX_COUNT][];

    public synchronized void addItem(Object obj) {
        if(counterForRow == ROW_MAX_COUNT) {
            objInstances[++rowCount] = new Object[ROW_MAX_COUNT];
            counterForRow = 0;
        }

        objInstances[rowCount][counterForRow++] = obj;
    }

    public synchronized  void clear() {
        objInstances = new Object[ROW_MAX_COUNT][];
        rowCount = 0;
        counterForRow = 0;
    }

    public synchronized Object getItem(int index) {
        int row = index / ROW_MAX_COUNT;
        int column = index % ROW_MAX_COUNT;

        if((row <= rowCount) && (column <= counterForRow)) {
            return objInstances[row][column];
        }
        return null;
    }
}

Answer 1

在您的代码中，clear 和getItem 是实例方法。将 synchronized 放在实例方法上意味着线程必须在调用该方法的对象实例上获取锁(“内部锁”)，然后线程才能开始执行该方法中的任何代码。

使实例方法同步有两个效果(来自 java 指南):

• First, it is not possible for two invocations of synchronized methods on the same object to interleave. When one thread is executing a synchronized method for an object, all other threads that invoke synchronized methods for the same object block (suspend execution) until the first thread is done with the object.
• Second, when a synchronized method exits, it automatically establishes a happens-before relationship with any subsequent invocation of a synchronized method for the same object. This guarantees that changes to the state of the object are visible to all threads.

因此，您的类对于这两个方法已经是线程安全的。