Vector 線程安全與不安全

下面這樣寫法是線程不安全的寫法

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刪除");
	                        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獲取");

	                        System.out.println((vector.get(i)));
	                    }
	                }
	            });

	            removeThread.start();
	            printThread.start();

	            //不要同時產生過多的線程，否則會導致操作系統假死
	           while (Thread.activeCount() > 20);
	        }
	    }
}

儘管Vector get()、remove()、get() 方法是I同步的 但運行上面程序會出現以下錯誤

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)
	at java.lang.Thread.run(Thread.java:722)

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獲取");
                     System.out.println((hashtable.get(i)));
                 }			
			}
		});
		   removeThread.start();
		   getThread.start();
          while (Thread.activeCount() > 20);
	     }
      }
}
</integer></integer>

會出現很多null值，但不錯，因爲沒有那個key ，但不會報錯

getThread獲取
null
getThread獲取
null

在多線程環境中，如果不在方法調用端做額外的同步措施，使用這段仍是線程不安全的，因爲如果一個線程恰好再錯誤的時間刪除了一個元素， 導致i不在可用的話，get方法會拋出一個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刪除");
	  	                        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獲取");

	                        System.out.println((vector.get(i)));
	                    }
	                	}
	                }
	            });

	            removeThread.start();
	            printThread.start();

	            //不要同時產生過多的線程，否則會導致操作系統假死
	           while (Thread.activeCount() > 20);
	        }
	      }
 }