synchronized(class)很特别,它会让另一个线程在任何需要获取class做为monitor的地方等待.class与this做为不同的监视器可以同时使用,不存在一个线程获取了class,另一个线程就不能获取该class的一切实例.
根据下面的代码自行修改,分别验证下面的几种情况:
synchronized(class)
synchronized(this)
->线程各自获取monitor,不会有等待.
synchronized(this)
synchronized(this)
->如果不同线程监视同一个实例对象,就会等待,如果不同的实例,不会等待.
synchronized(class)
synchronized(class)
->如果不同线程监视同一个实例或者不同的实例对象,都会等待.
package thread.sync;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class RunnableTest implements Runnable {
private static boolean flag = true;
// private static synchronized void testSyncMethod() { // 注意static修改的同步方法,监视器=class
// for (int i = 0; i < 100; i++) {
// System.out.println("testSyncMethod:" + i);
// }
// }
private void testSyncMethod() {
synchronized (this) {
for (int i = 0; i < 100; i++) {
System.out.println("testSyncMethod:" + i);
}
}
}
private void testSyncBlock() {
synchronized (this) { // 注意this做为监视器.它与class分别是二个不同监视器.不会存在class被获取,this就要等的现象.这也是我以前关于监视器的一个误区.
for (int i = 0; i < 100; i++) {
System.out.println("testSyncBlock:" + i);
}
}
// synchronized (RunnableTest.class) { // 显示使用获取class做为监视器.它与static synchronized method隐式获取class监视器一样.
// for (int i = 0; i < 100; i++) {
// System.out.println("testSyncBlock:" + i);
// }
// }
}
public void run() {
// flag是static的变量.所以,不同的线程会执行不同的方法,只有这样才能看到不同的锁定效果.
if (flag) {
flag = false;
testSyncMethod();
} else {
flag = true;
testSyncBlock();
}
}
public static void main(String[] args) {
ExecutorService exec = Executors.newFixedThreadPool(2);
RunnableTest rt = new RunnableTest();
RunnableTest rt1 = new RunnableTest();
exec.execute(rt);
exec.execute(rt1);
exec.shutdown();
}
}
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