局部变量是引用类型是否造成线程安全？

雨点打透心脏的1/2处 2022-12-02 04:28 100阅读 0赞

### 情形一：成员变量被多线程共享 ###

import java.util.ArrayList;
    import java.util.List;
    
    public class ThreadUnSafeTest {
        
    
        static final int THREAD_NUMBER = 2;
        static final int LOOP_NUMBER = 200;
        public static void main(String[] args) {
        
    		//同一个对象被多个线程共享
            final ThreadUnSafe test = new ThreadUnSafe();
    
            for (int i = 0; i < THREAD_NUMBER; i++) {
        
                new Thread(()->{
        
                    test.method1(LOOP_NUMBER);
                },"Thread" + (i + 1)).start();
            }
        }
    }
    
    class ThreadUnSafe{
        
    
        List<String> list = new ArrayList<String>();
    
        public void method1(int loopNumber){
        
            for (int i = 0; i < loopNumber; i++) {
        
                method2();
                method3();
            }
        }
    
        public void method2(){
        
            list.add("1");
        }
    
        public void method3(){
        
            list.remove(0);
        }
    }

此代码有可能造成以下异常：

Exception in thread "Thread1" java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
    	at java.util.ArrayList.rangeCheck(ArrayList.java:657)
    	at java.util.ArrayList.remove(ArrayList.java:496)
    	at ThreadUnSafe.method3(ThreadUnSafeTest.java:35)
    	at ThreadUnSafe.method1(ThreadUnSafeTest.java:26)
    	at ThreadUnSafeTest.lambda$main$0(ThreadUnSafeTest.java:13)
    	at java.lang.Thread.run(Thread.java:748)

原因：因为多个线程共享test对象中的list成员变量，同时对list进行add和remove操作造成的，`List是线程不安全的类`。

--------------------

### 情形二：使用局部变量list ###

public class ThreadUnSafeTest {
        
    
        static final int THREAD_NUMBER = 2;
        static final int LOOP_NUMBER = 200;
        public static void main(String[] args) {
        
            final ThreadSafe test = new ThreadSafe();
    
            for (int i = 0; i < THREAD_NUMBER; i++) {
        
                new Thread(()->{
        
                    test.method1(LOOP_NUMBER);
                },"Thread" + (i + 1)).start();
            }
        }
    }
    
    class ThreadSafe{
        
    
        public void method1(int loopNumber){
        
            //局部变量，list存在于每个线程的私有栈帧中，不会构成临界资源
            List<String> list = new ArrayList<String>();
            for (int i = 0; i < loopNumber; i++) {
        
                method2(list);
                method3(list);
            }
        }
    
        public void method2(List<String> list){
        
            list.add("1");
        }
    
        public void method3(List<String> list){
        
            list.remove(0);
        }
    }

运行此代码将不会出现错误。

原因：虽然多个线程共享test对象，但是每个线程在`栈帧`中拥有自己的list对象，`不构成临界区`，不会发成线程不安全问题。