请看这个代码
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 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，除了那次我使用了信号量(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呢？
这就引出了我的下一个问题。
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();
}
}
我假设由于在上面的 Main 类中创建了 20 个线程，因此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唯一的对象，因此同步方案实际上会失败，我是否正确？
换句话说，创建许多线程，每个线程都自行同步是一个坏主意吗？
PrintStream.println是synchronized所以即使你有很多线程，你的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();
}
}