java - 不同线程在不同对象上同步

Question

请看一下这段代码

public class Test extends Thread {
    int i;
    public Test(int i) {this.i = i;}

    void simpleBlock() throws InterruptedException {
        System.out.println(i + " this is an example of a thread blocking on itself - practicing concurrency 101, monitors, etc.");
        synchronized(this) {wait();}
    }

    public void run() {
        try {simpleBlock();} catch (InterruptedException e) {}
    }
}

这是由创建并启动线程的 Main 类实现的

public class Main {
    public static void main(String[] args) throws InterruptedException {
        Test[] t = new Test[20];
        for (int i=0; i<20; i++) {
            t[i] = new Test(i);
            t[i].start();
        }
    }
}

这将打印以下输出

0 this is an example of a thread blocking on itself - practicing concurrency 101, monitors, etc.
6 this is an example of a thread blocking on itself - practicing concurrency 101, monitors, etc.
4 this is an example of a thread blocking on itself - practicing concurrency 101, monitors, etc.
3 this is an example of a thread blocking on itself - practicing concurrency 101, monitors, etc.
5 this is an example of a thread blocking on itself - practicing concurrency 101, monitors, etc.
2 this is an example of a thread blocking on itself - practicing concurrency 101, monitors, etc.
1 this is an example of a thread blocking on itself - practicing concurrency 101, monitors, etc.
14 this is an example of a thread blocking on itself - practicing concurrency 101, monitors, etc.
7 this is an example of a thread blocking on itself - practicing concurrency 101, monitors, etc.
13 this is an example of a thread blocking on itself - practicing concurrency 101, monitors, etc.
12 this is an example of a thread blocking on itself - practicing concurrency 101, monitors, etc.
10 this is an example of a thread blocking on itself - practicing concurrency 101, monitors, etc.
11 this is an example of a thread blocking on itself - practicing concurrency 101, monitors, etc.
9 this is an example of a thread blocking on itself - practicing concurrency 101, monitors, etc.
8 this is an example of a thread blocking on itself - practicing concurrency 101, monitors, etc.
18 this is an example of a thread blocking on itself - practicing concurrency 101, monitors, etc.
19 this is an example of a thread blocking on itself - practicing concurrency 101, monitors, etc.
17 this is an example of a thread blocking on itself - practicing concurrency 101, monitors, etc.
16 this is an example of a thread blocking on itself - practicing concurrency 101, monitors, etc.
15 this is an example of a thread blocking on itself - practicing concurrency 101, monitors, etc.

由于 SimpleBlock() 不同步，我预计打印会被随机切碎。至少，这就是我不久前运行的另一个同步测试所发生的情况，除了那次我使用信号量(或缺乏信号量)。

那么为什么每个线程都以如此有序的方式打印完整的字符串？

这引导我进行下一个询问。

假设 simpleBlock 是完全同步的，输出与上面相同。所以，

    synchronized void simpleBlock() throws InterruptedException {
        System.out.println(i + " this is an example of a thread blocking on itself - practicing concurrency 101, monitors, etc.");
        wait();
    }

您可能已经知道，这相当于

    void simpleBlock() throws InterruptedException {
        synchronized(this) {
            System.out.println(i + " this is an example of a thread blocking on itself - practicing concurrency 101, monitors, etc.");
            this.wait();
        }
    }

我的假设是否正确，因为上面的 Main 类中创建了 20 个线程，所以没有两个线程在共享对象上同步，因为每个线程都是其自己的唯一对象，因此，同步方案实际上会失败？

换句话说，创建多个线程，每个线程自身同步是一个坏主意吗？

Answer 1

PrintStream.println 是同步的，因此即使您有很多线程，您的输出也相对有序。

来源

/**
 * Prints a String and then terminate the line.  This method behaves as
 * though it invokes <code>{@link #print(String)}</code> and then
 * <code>{@link #println()}</code>.
 *
 * @param x  The <code>String</code> to be printed.
 */
public void println(String x) {
    synchronized (this) {
        print(x);
        newLine();
    }
}