java.util.Collections.synchronizedMap()

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Description

The synchronizedMap() method is used to return a synchronized (thread-safe) map backed by the specified map.

Declaration

Following is the declaration for java.util.Collections.synchronizedMap() method.

public static <K,V> Map<K,V> synchronizedMap(Map<K,V> m)

Parameters

m − This is the map to be "wrapped" in a synchronized map.

Return Value

• The method call returns a synchronized view of the specified map.

Collections.synchronizedMap()与ConcurrentHashM两者都提供了线程同步的功能 

package com.ldd;

import java.util.*;
import java.util.concurrent.ConcurrentHashMap;

public class Functiontest {
    private static Map<String, Object> map1 = new HashMap<String, Object>();
    private static Map<String, Object> map2 = new Hashtable<String, Object>();
    private static Map<String, Object> map3 = new ConcurrentHashMap<String, Object>();
    private static Map<String, Object> map4 = Collections.synchronizedMap(new HashMap<String, Object>());

    private static Map<String, Object> map = map4;

    public static void main(String[] args) {
        new Thread(new Runnable() {

            @Override
            public void run() {
                while (true) {
                       if (map.size() > 0) {
                          for (Map.Entry<String, Object> entry : map.entrySet()) {
                             System.out.println(String.format("%s: %s", entry.getKey(), entry.getValue()));
                        }
                   // Iterator<Map.Entry<String, Object>> iterator = map4.entrySet().iterator();
                  //  while(iterator.hasNext()){
                 //       Map.Entry<String,Object>entry=iterator.next();
                  //      System.out.println("key= "+entry.getKey()+" , "+entry.getValue());
                   }
                    map.clear();
                    try {
                        Thread.sleep((new Random().nextInt(10) + 1) * 1);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }).start();

        new Thread(new Runnable() {

            @Override
            public void run() {

                for (int i = 1; i <= 100; i++) {
                    map.put("key" + i, "value" + i);
                    try {
                        Thread.sleep((new Random().nextInt(10) + 1) * 1);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }).start();
    }
}

Exception in thread "Thread-0" java.util.ConcurrentModificationException   

Collections.synchronizedMap() guarantees that each atomic operation you want to run on the map will be synchronized.

Running two (or more) operations on the map however, must be synchronized in a block.

使用synchronizedMap() 没报错，但缺失一部分数据

 

加上锁：

package com.ldd;

import java.util.*;
import java.util.concurrent.ConcurrentHashMap;

public class Functiontest {
    private static Map<String, Object> map1 = new HashMap<String, Object>();
    private static Map<String, Object> map2 = new Hashtable<String, Object>();
    private static Map<String, Object> map3 = new ConcurrentHashMap<String, Object>();
    private static Map<String, Object> map4 = Collections.synchronizedMap(new HashMap<String, Object>());

    private static Map<String, Object> map = map3;
    private static int count=0;
    public static void main(String[] args) {
        new Thread(new Runnable() {

            @Override
            public void run() {

                for (int i = 1; i <= 100; i++) {
                    synchronized (map) {
                        map.put("key" + i, "value" + i);
                    }
                    try {
                        Thread.sleep((new Random().nextInt(10) + 1) * 1);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }).start();
        new Thread(new Runnable() {

            @Override
            public void run() {
                while (true) {
                    synchronized (map) {
                       if (map.size() > 0) {
                           for (Map.Entry<String, Object> entry : map.entrySet()) {
                               count++;
                               System.out.println(String.format("%s: %s %d", entry.getKey(), entry.getValue(), count));
                           }
                       }
                   // Iterator<Map.Entry<String, Object>> iterator = map4.entrySet().iterator();
                  //  while(iterator.hasNext()){
                 //       Map.Entry<String,Object>entry=iterator.next();
                  //      System.out.println("key= "+entry.getKey()+" , "+entry.getValue());
                        map.clear();
                    }

                    try {
                        Thread.sleep((new Random().nextInt(10) + 1) * 1);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }).start();


    }
}

https://www.tutorialspoint.com/java/util/collections_synchronizedmap.htm