java併發編程面試題1

實現一個容器，提供兩個方法add和size
寫兩個線程，線程1添加十個元素到容器中，線程2實現監控元素的個數，當個數達到五個時線程2 給出提示並結束

方法一：volatile關鍵字

public class MyContainer1 {
	volatile List list = new ArrayList();
	
	public void add(Object o) {
		list.add(o);
	}
	
	public int size() {
		return list.size();
	}
	
	
	public static void main(String[] args) {
		
		MyContainer1 container1 = new MyContainer1();
		
		new Thread(() -> {
			for(int i = 0; i < 10; i++) {
				container1.add(new Object());
				System.err.println("add" + i);
				
				try {
					TimeUnit.SECONDS.sleep(1);
				} catch (Exception e) {
					// TODO: handle exception
					e.printStackTrace();
				}
			}
		}, "t1").start();
		
		new Thread(() -> {
			while(true)
				if(container1.size() == 5)
					break;
			System.err.println("t2 end");
		}, "t2").start();
	}
}

缺點：
while循環太費CPU

2.使用wait和notify方法<這種方法不行，這裏只是做一個演示>

• wait方法會釋放鎖
• notify不會釋放鎖

public class MyContainer2 {
	volatile List list = new ArrayList();
	
	public void add(Object o) {
		list.add(o);
	}
	
	public int size() {
		return list.size();
	}
	
	
	public static void main(String[] args) {
		
		MyContainer2 container1 = new MyContainer2();
		
		final Object lock = new Object();
		
		new Thread(() -> {
			synchronized (lock) {
				System.out.println("t2 start");
				
				if(container1.size() != 5) {
					try {
						lock.wait();
					} catch (InterruptedException e) {
						// TODO: handle exception
					}
				}
				System.out.println("t2 end...");
			}
				
		}, "t2").start();
		
		new Thread(() -> {
			synchronized (lock) {
				for(int i = 0; i < 10; i++) {
					container1.add(new Object());
					System.err.println("add" + i);
					
					if (container1.size() == 5) {
						lock.notify();//會喚醒線程，但是不會釋放鎖，所以t2還是不會有鎖，所以還是會出錯
					}
					try {
						TimeUnit.SECONDS.sleep(1);
					} catch (Exception e) {
						// TODO: handle exception
						e.printStackTrace();
					}
				}
			}
		}, "t1").start();
		
		
	}
}

調整如下：

public class MyContainer3 {
	volatile List list = new ArrayList();
	
	public void add(Object o) {
		list.add(o);
	}
	
	public int size() {
		return list.size();
	}
	
	
	public static void main(String[] args) {
		
		MyContainer3 container1 = new MyContainer3();
		
		final Object lock = new Object();
		
		new Thread(() -> {
			synchronized (lock) {
				System.out.println("t2 start");
				
				if(container1.size() != 5) {
					try {
						lock.wait();
					} catch (InterruptedException e) {
						// TODO: handle exception
					}
				}
				System.out.println("t2 end...");

				lock.notify();
			}
				
		}, "t2").start();
		
		new Thread(() -> {
			synchronized (lock) {
				for(int i = 0; i < 10; i++) {
					container1.add(new Object());
					System.err.println("add" + i);
					
					if (container1.size() == 5) {
						lock.notify();//會喚醒線程，但是不會釋放鎖，所以t2還是不會有鎖
						
						try {
							lock.wait();
						} catch (Exception e) {
							// TODO: handle exception
						}
					}
					try {
						TimeUnit.SECONDS.sleep(1);
					} catch (Exception e) {
						// TODO: handle exception
						e.printStackTrace();
					}
				}
			}
		}, "t1").start();
	}
}

方法三：

• 使用latch中的await和countdown來替代wait和notify
• 好處是：通信簡單，同時可以指定等待的時間
• CountDownLatch不涉及鎖定，當count值爲0 時當前的線程繼續執行
• 這並不涉及同步，只是在線程通信的過程中synchronized + wait/notify 顯得太重了

public class MyContainer4 {
	volatile List list = new ArrayList();
	
	public void add(Object o) {
		list.add(o);
	}
	
	public int size() {
		return list.size();
	}
	
	
	public static void main(String[] args) {
		
		MyContainer4 container1 = new MyContainer4();
		
		CountDownLatch latch = new CountDownLatch(1);
		
		new Thread(() -> {
				System.out.println("t2 start");
				
				if(container1.size() != 5) {
					try {
						latch.await();
					} catch (InterruptedException e) {
						// TODO: handle exception
					}
				}
				System.out.println("t2 end...");			
				
		}, "t2").start();
		
		new Thread(() -> {
				for(int i = 0; i < 10; i++) {
					container1.add(new Object());
					System.err.println("add" + i);
					
					if (container1.size() == 5) {
						latch.countDown();
					}
					try {
						TimeUnit.SECONDS.sleep(1);
					} catch (Exception e) {
						// TODO: handle exception
						e.printStackTrace();
					}
				}
			
		}, "t1").start();
		
		
	}
}