在不同对象上同步的不同线程

Question

请看这段代码

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) {}
    }
}

这是由创建和启动线程的主类实现的。

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()是不同步的，所以我希望这些打印文件会被随机剪切。至少，这是我在一段时间前运行的另一个同步测试中发生的情况，但当时我使用的是信号量(或者没有信号量)。

，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呢？

这就引出了我的下一个调查。

假设simpleBlock是完全同步的，并且输出与上面相同。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();
        }
    }

我是否正确地假设，由于在上面的主类中创建了20个线程，没有两个线程在共享对象上同步，因为每个线程都是它自己的唯一对象，因此同步方案实际上会失败吗？

换句话说，创建多个线程(每个线程在自身上同步)是不是一个坏主意？

Answer 1

PrintStream.println是synchronized，所以即使有很多线程，输出也是相对有序的。

从源头

/**
 * 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();
    }
}