哪些集合对象线程是安全的，哪些是不安全的。

来源：互联网发布：php开源做账系统编辑：程序博客网时间：2024/06/08 20:08

 - 首先给出结论，ArrayList，LinkedList，HashMap,StringBuilder是不安全的。vector,HashTable,StringBuffer是线程安全的。下面用代码来看看。

ArrayList测试

package testThread;import java.util.ArrayList;import java.util.List;class Ticket implements  Runnable {    private static int t=0;    List<Integer> data = new ArrayList<>();    boolean flag=true;      public void run() {         while (flag) {            if (t<100) {                  data.add(t);                  t++;                  //System.out.println(t);                    }else {                                                                     flag=false;                        System.out.println("数据宽度"+data.size());                                                         for (int i = 0; i < data.size(); i++) {                            System.out.println(data.get(i));                        }                    }                   }    }   }public class thread1 {     public static  void main(String[] args)     {        Ticket t=new Ticket();        Thread t1=new Thread(t);           Thread t2=new Thread(t);        Thread t3=new Thread(t);        t1.start();        t2.start();             t3.start();     }}

ArrayList测试结果
这里写图片描述
2.LinkedList

package te;import java.util.LinkedList;import java.util.List;class Ticket implements  Runnable {    private static int t=0;    List<Integer> data = new LinkedList<Integer>();    boolean flag=true;      public void run() {         while (flag) {            if (t<100) {                  data.add(t);                  t++;                  //System.out.println(t);                    }else {                                                                     flag=false;                        System.out.println("数据宽度"+data.size());                                                         for (int i = 0; i < data.size(); i++) {                            System.out.println(data.get(i));                        }                    }                   }    }   }public class Thread7 {     public static  void main(String[] args)     {        Ticket t=new Ticket();        Thread t1=new Thread(t);           Thread t2=new Thread(t);        Thread t3=new Thread(t);        t1.start();        t2.start();             t3.start();     }}

LinkedList结果
这里写图片描述

3.HashMap

package te;import java.util.Collection;import java.util.HashMap;import java.util.Iterator;import java.util.List;import java.util.Map;class Ticket implements  Runnable {    private static int t=0;    Map data = new HashMap<Integer,Integer>();    boolean flag=true;      public void run() {         while (flag) {            if (t<100) {                  data.put(t,t);                  t++;                  //System.out.println(t);                    }else {                                                                     flag=false;                                                                                 Collection da=data.values();                        Iterator<Integer> it=da.iterator();                        while (it.hasNext()) {                            Integer integer = (Integer) it.next();                            System.err.println(integer);                        }                    }                   }    }   }public class Thread7 {     public static  void main(String[] args)     {        Ticket t=new Ticket();        Thread t1=new Thread(t);           Thread t2=new Thread(t);        Thread t3=new Thread(t);        t1.start();        t2.start();             t3.start();     }}

HashMap测试
这里写图片描述
4.vector

package te;import java.util.Vector;class Ticket implements  Runnable {    private static int t=0;    Vector<Integer> vector=new Vector<Integer>();    boolean flag=true;      public void run() {         while (flag) {            if (t<100) {                  vector.add(t);                  t++;                                }else {                                                                     flag=false;                                                                                 for(int i=0;i<vector.size();i++)                        {                            System.out.println(vector.get(i));                        }                    }                   }    }   }public class Thread7 {     public static  void main(String[] args)     {        Ticket t=new Ticket();        Thread t1=new Thread(t);           Thread t2=new Thread(t);        Thread t3=new Thread(t);        t1.start();        t2.start();             t3.start();     }}

vector线程是安全的
5.stringbuilder和stringbuffer

package te;class Ticket implements  Runnable {    private static int t=0;     StringBuffer sb = new StringBuffer();       StringBuilder sc=new StringBuilder();    boolean flag=true;       public void run() {            while (flag) {            if (t<100) {                  sb.append(t+"a ");                  sc.append(t+"a ");                  t++;                                }else {                                                                     flag=false;                                                                                 System.out.println("stringbuffer"+sb);                        System.out.println("stringbuilder"+sc);                    }                   }    }   }public class Thread7 {     public static  void main(String[] args)     {        Ticket t=new Ticket();        Thread t1=new Thread(t);           Thread t2=new Thread(t);        Thread t3=new Thread(t);        t1.start();        t2.start();             t3.start();     }}

结果测试 stringbuilder是不安全的
这里写图片描述

6.分析总结
凡是线程不安全的,我们可以来看源码来分析，这是arraylist的源码，我们很容易发现这是由于没有synchronized关键字。

 public boolean add(E e) {        ensureCapacityInternal(size + 1);  // Increments modCount!!        elementData[size++] = e;        return true;    }

再来看看vector,有synchronized关键字。

 public synchronized boolean add(E e) {        modCount++;        ensureCapacityHelper(elementCount + 1);        elementData[elementCount++] = e;        return true;    }

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