一个类，有新增元素（add）和获取元素数量（size）方法。 启动两个线程。线程1向容器中新增数据。线程2监听容器元素数量，当容器元素数量为5时，线程2输出信息并终止

方式一：

/**
 * 两个线程要是可见的所以要加上votalile
 */public class Test_01 {
    public static void main(String[] args) {
        final Test_01_Container t = new Test_01_Container();
        new Thread(new Runnable() {
            @Override
            public void run() {
                for(int i = 0; i < 10; i++){
                    System.out.println("add Object to Container " + i);
                    t.add(new Object());
                    try {
                        TimeUnit.SECONDS.sleep(1);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }).start();
        
        new Thread(new Runnable(){
            @Override
            public void run() {
                while(true){
                    if(t.size() == 5){
                        System.out.println("size = 5");
                        break;
                    }
                }
            }
        }).start();
    }
}

class Test_01_Container{
    volatile List<Object> container = new ArrayList<>();
    
    public void add(Object o){
        this.container.add(o);
    }
    
    public int size(){
        return this.container.size();
    }
}

方式二：

/**
 * wait notify
 */
package concurrent.t02;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.TimeUnit;

public class Test_02 {
    public static void main(String[] args) {
        final Test_02_Container t = new Test_02_Container();
        final Object lock = new Object();
        
        new Thread(new Runnable(){
            @Override
            public void run() {
                synchronized (lock) {
                    if(t.size() != 5){
                        try {
                            lock.wait(); // 线程进入等待队列。
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    System.out.println("size = 5");
                    lock.notifyAll(); // 唤醒其他等待线程
                }
            }
        }).start();
        
        new Thread(new Runnable() {
            @Override
            public void run() {
                synchronized (lock) {
                    for(int i = 0; i < 10; i++){
                        System.out.println("add Object to Container " + i);
                        t.add(new Object());
                        if(t.size() == 5){
                            lock.notifyAll();
                            try {
                                lock.wait();
                            } catch (InterruptedException e) {
                                e.printStackTrace();
                            }
                        }
                        try {
                            TimeUnit.SECONDS.sleep(1);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }
            }
        }).start();
    }
}

class Test_02_Container{
    List<Object> container = new ArrayList<>();
    
    public void add(Object o){
        this.container.add(o);
    }
    
    public int size(){
        return this.container.size();
    }
}

方式三：

/**
 * CountDownLatch 门闩
 */
package concurrent.t02;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.Semaphore;
import java.util.concurrent.TimeUnit;

public class Test_03 {
    public static void main(String[] args) {
        final Test_03_Container t = new Test_03_Container();
        final CountDownLatch latch = new CountDownLatch(1);

        new Thread(new Runnable(){
            @Override
            public void run() {
                if(t.size() != 5){
                    try {
                        latch.await(); // 等待门闩的开放。 不是进入等待队列
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                System.out.println("size = 5");
            }
        }).start();
        
        new Thread(new Runnable() {
            @Override
            public void run() {
                for(int i = 0; i < 10; i++){
                    System.out.println("add Object to Container " + i);
                    t.add(new Object());
                    if(t.size() == 5){
                        latch.countDown(); // 门闩-1
                    }
                    try {
                        TimeUnit.SECONDS.sleep(1);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }).start();
    }
}

class Test_03_Container{
    List<Object> container = new ArrayList<>();
    
    public void add(Object o){
        this.container.add(o);
    }
    
    public int size(){
        return this.container.size();
    }
}