Java tread Multithreading (2) Multithreading security issues

Author: Ching

Original address: http://blog.csdn.net/qingdujun/article/details/39348093

This article demonstrates the tread multithreading security issue, as well as a multi-threaded security approach (thread synchronization).

1) A thread unsafe demo

2) thread synchronization

A small demo demo leads to thread safety issues:

Package THREAD.RUNABLE1.QDJ;//1. Defines the class implementation Runnable interface class RUNDEMO1 implements runnable{private int x = 5;//2. Overwrite the Run method in the Runnable interface//to store the thread code in run public void run () {while (true) {if (x > 0) {//Add sleep (), note: Sleep () throws an exception try { Thread.Sleep (ten);  Let the thread Sleep 10ms} catch (Exception e) {e.printstacktrace ();} System.out.println ("Runnable:" +x);--x;}}} public class CRunableDemo1 {public static void main (string[] args) {RunDemo1 r = new RunDemo1 ()//3. Thread objects are established through the thread class and Runn The subclass object of the able interface as the parameter Thread t1 = new Thread (r); Thread t2 = new Thread (r); thread t3 = new Thread (r); thread T4 = new Thread (r),//4. Using start to open thread T1.start (); T2.start (); T3.start (); T4.start ();}}

Run Display results:

The above print out the 0,-1,-2 and other figures, found not? (Threading Problem!!!) ）
What is the cause of the problem?

Thread 1 entered, forced to sleep 10ms, the thread 2 entered, and forced to sleep 10ms, thread 3 entered and forced sleep 10ms, thread 4 into the forced sleep 10ms;

Note that the above 4 threads sleep have entered the IF statement, when entering the time x>0 is still established;

Okay, thread 1 wakes up, starts printing print 5,4,3,2, when--x is not executed, thread 2 wakes up, grabs the CPU execution right ............

Second, thread synchronization

Question: Can we take one such measure for the above question? When thread 1 executes the run code snippet, we do not allow other threads to execute until thread 1 executes and the other threads can enter.

Solution: Fortunately, Java has such a function, the code snippet wrapped up, you can achieve the above problem description of the effect. Function name: synchronized, need a parameter, just pass an object is OK.

Package THREAD.RUNABLE1.QDJ;//1. Define class implementation Runnable interface class RUNDEMO1 implements runnable{private int x = 5;object obj = new Objec T ();//2. Overwrite the Run method in the Runnable interface//Store thread code in run public void run () {while (true) {synchronized (obj) {if (x > 0) {//Add sleep (), Note: Sleep () throws an exception to try {Thread.Sleep (ten);  Let the thread Sleep 10ms} catch (Exception e) {e.printstacktrace ();} System.out.println ("Runnable:" +x);--x;}}}} public class CRunableDemo1 {public static void main (string[] args) {RunDemo1 r = new RunDemo1 ()//3. Thread objects are established through the thread class and Runn The subclass object of the able interface as the parameter Thread t1 = new Thread (r); Thread t2 = new Thread (r); thread t3 = new Thread (r); thread T4 = new Thread (r),//4. Using start to open thread T1.start (); T2.start (); T3.start (); T4.start ();}}

The running results show:

References: Java video Bi Xiangdong presenter

Original address: http://blog.csdn.net/qingdujun/article/details/39348093

Java tread Multithreading (2) Multithreading security issues