CountDownLatch测试，并发测试，主线程等待子线程全部执行完后执行测试

桃扇骨 2023-10-06 14:29 38阅读 0赞

1、测试下面结果按理说应该为1000000，但是因为并发导致结果错误。

package demo;
    
    import java.util.concurrent.CountDownLatch;
    
    public class TestCountDownLatch {
    	private static int N = 1000; // 线程数
    	private static int m = 0;
    
    	public static void main(String[] args) throws Exception {
    		CountDownLatch countDownLatch = new CountDownLatch(N);// 实例化一个倒计数器，N指定计数个数
    		for (int i = 0; i < N; i++) {
    			new Thread(() -> {
    				try {
    					// System.out.println(Thread.currentThread().getName() + "启动@" + System.currentTimeMillis());
    					for (int j = 0; j < 1000; j++) {
    						m++;
    					}
    				} catch (Exception e) {
    					e.printStackTrace();
    				} finally {
    					countDownLatch.countDown(); // 计数减一
    				}
    			}).start();
    
    		}
    		countDownLatch.await();// 等待，当计数减到0时，所有线程并行执行
    		System.out.println(m); // 结果应该1000000
    	}
    }

2、上面的问题加锁解决。

等待所有子线程结束，主线程再执行测试。

package demo;
    
    import java.util.concurrent.CountDownLatch;
    
    public class TestCountDownLatch {
    	private static int N = 1000; // 线程数
    	private static int m = 0;
    
    	public static void main(String[] args) throws Exception {
    		Object o = new Object();
    		CountDownLatch countDownLatch = new CountDownLatch(N);// 实例化一个倒计数器，N指定计数个数
    		for (int i = 0; i < N; i++) {
    			new Thread(() -> {
    				try {
    					// System.out.println(Thread.currentThread().getName() + "启动@" + System.currentTimeMillis());
    					for (int j = 0; j < 1000; j++) {
    						synchronized (o) {// 普通方法上锁为this,静态static方法上锁为类名.class
    							m++;
    						}
    					}
    				} catch (Exception e) {
    					e.printStackTrace();
    				} finally {
    					countDownLatch.countDown(); // 计数减一
    				}
    			}).start();
    
    		}
    		countDownLatch.await();// 等待，当计数减到0时，所有线程并行执行
    		System.out.println(m); // 结果应该1000000
    	}
    }

3、等待所有线程加载完，同时并发执行某个代码块。

package demo.teststatic;
    
    import java.util.concurrent.CountDownLatch;
    
    public class Test2 {
    
    	private static int N = 10000; // 线程数
    	private static int m = 0;
    
    	public static void main(String[] args) throws Exception {
    		CountDownLatch countDownLatch = new CountDownLatch(N);// 实例化一个倒计数器，N指定计数个数
    		for (int i = 0; i < N; i++) {
    			new Thread(() -> {
    				try {
    					countDownLatch.countDown(); // 计数减一
    					System.out.println(m++);
    					countDownLatch.await();// 等待，当计数减到0时，所有线程并行执行
    					System.out.println("开始");
    					for (int j = 0; j < 10000; j++) {
    						//执行代码块
    					}
    				} catch (Exception e) {
    					e.printStackTrace();
    				} finally {
    
    				}
    			}).start();
    		}
    	}
    }