VectorHashTable线程不安全示例

Posted 2022-06-13 jing99

下面这样写法是Vector线程不安全的写法：

import java.util.Vector;
 
public class Test  
     private static Vector<Integer> vector = new Vector<Integer>();
 
        public static void main(String[] args) 
            while (true) 
                for (int i = 0; i < 10; i++) 
                    System.out.println("添加");
                    vector.add(i);
                
 
                Thread removeThread = new Thread(new Runnable() 
                    @Override
                    public void run() 
                        for (int i = 0; i < vector.size(); i++) 
                            System.out.println("removeThread删除");
                            vector.remove(i);
                        
                    
                );
 
                Thread printThread = new Thread(new Runnable() 
                    @Override
                    public void run() 
                        for (int i = 0; i < vector.size(); i++) 
                            System.out.println("printThread获取");
 
                            System.out.println((vector.get(i)));
                        
                    
                );
 
                removeThread.start();
                printThread.start();
 
                //不要同时产生过多的线程，否则会导致操作系统假死
               while (Thread.activeCount() > 20);
            
        
　　

　　尽管Vector get()、remove()、get() 方法是I同步的 但运行上面程序会出现以下错误:

java.lang.ArrayIndexOutOfBoundsException: Array index out of range: 0
    at java.util.Vector.get(Vector.java:744)
    at Test$2.run(Test.java:29)
    at java.lang.Thread.run(Thread.java:722)
Exception in thread "Thread-14857" java.lang.ArrayIndexOutOfBoundsException: Array index out of range: 0
    at java.util.Vector.get(Vector.java:744)
    at Test$2.run(Test.java:29)

HashTable线程不安全写法：

import java.util.Hashtable;
import java.util.Map;
 
public class HashmapTest 
    
     private  static Map<Integer,Integer> hashtable= new Hashtable<Integer,Integer>();
      public static void main(String[] args) 
       while(true)
           for (int i = 0; i < 10; i++) 
                   System.out.println("添加");
                 hashtable.put(i, i);
           
           Thread removeThread = new Thread(new Runnable() 
            @Override
            public void run() 
             Iterator it = hashtable.entrySet().iterator();
                  while (it.hasNext()) 
                  Map.Entry<integer integer=""> entry=(Entry<integer integer="">) it.next();  
                           System.out.println("delete this: "+entry.getKey()+"==="+entry.getValue());  
                            it.remove(); 
                 
                    
                     
           
           Thread  getThread = new Thread(new Runnable() 
            @Override
            public void run() 
                 for (int i = 0; i < hashtable.size(); i++) 
                     System.out.println("getThread获取");
                     System.out.println((hashtable.get(i)));
                             
            
        );
           removeThread.start();
           getThread.start();
          while (Thread.activeCount() > 20);
         
      

　　会出现很多null值，但不错，因为没有那个key ，但不会报错

getThread获取
null
getThread获取
null

　　在多线程环境中，如果不在方法调用端做额外的同步措施，使用这段仍是线程不安全的，因为如果一个线程恰好再错误的时间删除了一个元素， 导致i不在可用的话，get方法会抛出一个ArrayIndexOutOfBoundsException

import java.util.Vector;
 
public class Test  
     private static Vector<Integer> vector = new Vector<Integer>();
 
        public static void main(String[] args) 
            while (true) 
                for (int i = 0; i < 10; i++) 
                    System.out.println("添加");
                    vector.add(i);
                
 
                Thread removeThread = new Thread(new Runnable() 
                    @Override
                    public void run() 
                        synchronized (vector) 
                              for (int i = 0; i < vector.size(); i++) 
                                  System.out.println("removeThread删除");
                                  vector.remove(i);
                              
                        
                    
                );
 
                Thread printThread = new Thread(new Runnable() 
                    @Override
                    public void run() 
                        synchronized (vector) 
                        for (int i = 0; i < vector.size(); i++) 
                            System.out.println("printThread获取");
 
                            System.out.println((vector.get(i)));
                        
                        
                    
                );
 
                removeThread.start();
                printThread.start();
 
                //不要同时产生过多的线程，否则会导致操作系统假死
               while (Thread.activeCount() > 20);