Vector、HashTable线程不安全示例

发布时间：2020-12-15 05:26:50 所属栏目：Java 来源：网络整理

导读：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 VectorInteger vector = new VectorInteger (); public static void main(String[] args) { while ( true ) { for ( int i = 0; i 10; i++ ) { System

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);
            }
      }
 }

（编辑：李大同）

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