线程上下文切换

public class LockSupportTest {

    public static void main(String[] args) throws InterruptedException {
        contextSwitchTest();
    }

    public static long count = 0;
    
    //
    public static void contextSwitchTest() {
        long start = System.currentTimeMillis();
        // 启动10000个线程，表示有10000个线程会来回上下文切换
        for (int i = 0; i < 10000; i++) {
            new Thread(() -> {
                synchronized (LockSupportTest.class) {
                    for (int j = 0; j < 10000; j++) {
                        count++;
                    }
                }
            }, "AAA").start();
        }
        // 主线程 + 后台gc线和，所以这里判断条件为2
        while (Thread.activeCount() > 2) {
            Thread.yield();
        }
        long end = System.currentTimeMillis();
        System.out.println("最终结果：" + count + "共执行：" + (end - start) + "ms"); // 最终结果：100000000共执行：539ms
    }

    public static void synchronizedTest() {
        long start = System.currentTimeMillis();
        for (int i = 0; i < 10000 * 10000; i++) {
            synchronized (LockSupportTest.class) {
                count++;
            }
        }
        long end = System.currentTimeMillis();
        System.out.println("最终结果：" + count + "共执行：" + (end - start) + "ms"); // 最终结果：100000000共执行：567ms
    }

    public static void noSwitchtest() {
        long start = System.currentTimeMillis();
        for (int i = 0; i < 10000 * 10000; i++) {
            count++;
        }
        long end = System.currentTimeMillis();
        System.out.println("最终结果：" + count + "共执行：" + (end - start) + "ms"); // 最终结果：100000000共执行：4ms
    }
}