Vector 线程安全与不安全

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下面这样写法是线程不安全的写法

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: 0at 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: 0at 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() {@Overridepublic 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() {@Overridepublic 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);     }      }}</integer></integer>

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);        }      } }

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