Research on Android multithreading (4) -- from an interview question

Hello everyone , I meet you again , I'm the king of the whole stack , I wish every programmer can learn more languages .

There is one such Interview questions ： Turn on a child thread to run at the same time as the main thread , Child thread output 10 Next, the main thread outputs 100 Time , repeat 50 Time . First look at the following code ：

package com.maso.test;

/**
 * 
 * @author Administrator
 *  Two threads , There is a main thread , The first thread runs the output first 10 Time , The main thread then runs the output 100 Time , repeat 50 Time 
 */
public class ThreadTest3 implements Runnable{
	private static Test test;
	@Override
	public void run() {
		for(int i=0; i<50; i++){
			test.f1(i);
		}
	}
	
	public static void main(String[] args) {
		test = new Test();
		new Thread(new ThreadTest3()).start();
		for(int i=0; i<50; i++){
			test.f2(i);
		}
	}
	
	/**
	 *  Classify control and logic and data （ This class is data ）
	 * @author Administrator
	 *
	 */
	static class Test{
		private boolean isf1 = true;
		/**
		 *  Output 10 Time 
		 */
		public synchronized void f1(int j){
			if(!isf1){
				try {
					this.wait();
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
			}
			for(int i=1; i<=10; i++){
				System.out.println(Thread.currentThread().getName() + " The first " + j + " Second round patrol , Output " + i);
			}
			isf1 = false;
			notify();
		}
		
		/**
		 *  Output 100 Time 
		 */
		public synchronized void f2(int j){
			if(isf1){
				try {
					this.wait();
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
			}
			for(int i=1; i<=100; i++){
				System.out.println(Thread.currentThread().getName() + " The first " + j + " Second round patrol , Output " + i);
			}
			isf1 = true;
			notify();
		}
	}
}

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The above inference uses if sentence , There seems to be no problem doing so , In fact, it is not safe to do so , Because the thread may be falsely awakened while waiting , So we need to use while sentence . In addition, we are using wait and notify There are several points to pay attention to when ：

1、 call object Of wait Methods and notity When the method is used , You have to get object The object of the lock （ Must be written in synchronized in ）.

2、 Suppose you call object Of wait Method , Then the thread releases the object lock .

3、 hypothesis A1、A2、A3 All in object.wait(), be B call object.notify（） Can only wake up A1、A2、A3 One of them （ Which one is detailed by JVM decision ）

4、object.notifyAll() Can wake up all .

5、B Wake up A When ,B Suppose you still hold the object lock , Wait until B After releasing the lock ,A To have a chance to run .

Sleep and Wait What's the difference ？

sleep（） Does not release the object lock ,wait（） Release object lock . But wait() and sleep() Can pass through interrupt() Method to break the thread's pause state , This causes the thread to immediately throw InterruptedException. Assuming that thread A Want to end the thread immediately B, Can be applied to threads B Corresponding Thread The instance interrupt Method . Suppose that at this moment, the thread B is wait/sleep/join, The thread B Will throw it right away InterruptedException, stay catch() {} In the direct return The thread can end safely . It should be noted that ,InterruptedException It's the thread itself that throws it from the inside , Is not interrupt() Method . Call... On a thread interrupt() when , Suppose the thread is running normal code , Then the thread will not throw InterruptedException. But , Once the thread enters wait()/sleep()/join() after , Will immediately throw out InterruptedException.

Let's take a look at the life cycle of threads ：

The method to realize thread scheduling is as follows ：

1、sleep(): This thread is to let the thread sleep for a certain time , Need to capture InterruptedException

2、yield(): Pause current thread , Let threads with the same priority run , Assuming that there is no thread with the same priority, it will not work . When it works, it will give way CPU The elapsed time , Get ready .

3、join(): Let a thread wait for the call join Method, and then continue to run .

Look at a piece of code ：

public class ThreadTest4 implements Runnable{
	private static int a = 0;
	@Override
	public void run() {
		for(int i=0; i<10; i++){
			a++;
		}
	}

	public static void main(String[] args) {
		new Thread(new ThreadTest4()).start();
		System.out.println(a);
	}
}

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This code will output 10 Do you ？ The answer is no , Because after starting the child thread , Immediately output a Value , At this time, the sub thread pair a No operation yet . Changes such as the following ：

public class ThreadTest4 implements Runnable{
	private static int a = 0;
	@Override
	public void run() {
		for(int i=0; i<10; i++){
			a++;
		}
	}

	public static void main(String[] args) throws InterruptedException {
		Thread t = new Thread(new ThreadTest4());
		t.start();
		t.join();
		System.out.println(a);
	}
}

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This time output 10,join() The role of methods can be seen , It will make other threads wait for the thread to finish running before running .

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