HashMap的高级用法：同步

HashMap是不支持同步的，不能用于多线程中。而HashTable是同步的，这个日后再论。我们可以使用以下两种方法来实现HashMap的同步：

• 使用ConcurrentHashMap

• 使用Collections.synchronizedMap获取同步map

使用ConcurrentHashMap

    ConcurrentHashMap<Integer,String> concurrentHashMap = new ConcurrentHashMap<>();

使用Collections.synchronizedMap获取同步map

    HashMap<Integer,String> hashMap = new HashMap<>();
    Map<Integer,String> synchronizedMap = Collections.synchronizedMap(hashMap);

测试

对于没有同步的HashMap在多线程中运行一定会抛出异常：

    java.util.ConcurrentModificationException

运行两个线程同时对一个hashmap进行读写操作：

    HashMap<Integer,String> hashMap1 = new HashMap<>();
        //写线程
        new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 1000; i++) {
                    hashMap1.put(i,"value"+i);
                    try {
                        Thread.sleep(new Random().nextInt(10));
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }).start();
        //读线程
        new Thread(new Runnable() {
            @Override
            public void run() {
                while (true) {
                    if (hashMap1.size() > 0) {
                        for (Map.Entry entry : hashMap1.entrySet()) {
                            System.out.println(entry.getKey() + "=" + entry.getValue());
                        }
                    }
                    try {
                        Thread.sleep(new Random().nextInt(10));
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }).start();

运行以上代码抛出异常java.util.ConcurrentModificationException。原因是未实现同步时，在迭代hashmap的时候不能修改数据。

Collections.synchronizedMap实现同步：

    HashMap<Integer,String> hashMap1 = new HashMap<>();
        Map<Integer, String> map = Collections.synchronizedMap(hashMap1);
        //写线程
        new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 1000; i++) {
                    map.put(i,"value"+i);
                    try {
                        Thread.sleep(new Random().nextInt(10));
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }).start();
        //读线程
        new Thread(new Runnable() {
            @Override
            public void run() {
                while (true) {
                    if (map.size() > 0) {
                        for (Map.Entry entry : map.entrySet()) {
                            System.out.println(entry.getKey() + "=" + entry.getValue());
                        }
                    }
                    try {
                        Thread.sleep(new Random().nextInt(10));
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }).start();

运行以上代码任然抛出异常java.util.ConcurrentModificationException。可见该方法并不安全。（还是这样测试有问题？）

ConcurrentHashMap实现同步：

      ConcurrentHashMap<Integer,String> concurrentHashMap = new ConcurrentHashMap<>();
       //写线程
        new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 1000; i++) {
                    concurrentHashMap.put(i,"value"+i);
                    try {
                        Thread.sleep(new Random().nextInt(10));
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }).start();
        //读线程
        new Thread(new Runnable() {
            @Override
            public void run() {
                while (true) {
                    if (concurrentHashMap.size() > 0) {
                        for (Map.Entry entry : concurrentHashMap.entrySet()) {
                            System.out.println(entry.getKey() + "=" + entry.getValue());
                        }
                    }
                    try {
                        Thread.sleep(new Random().nextInt(10));
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }).start();

运行以上代码，无异常。

所以为了保证多线程下的hashmap的数据一致性使用ConcurrentHashMap最为合理。并且ConcurrentHashMap的效率也比较高。为什么日后再论。

使用lock锁手动实现hashmap的同步：

     ReentrantLock lock = new ReentrantLock(true);
     HashMap<Integer, String> hashMap1 = new HashMap<>();

            //写线程
            new Thread(new Runnable() {
                @Override
                public void run() {
                    try {
                    for (int i = 0; i < 1000; i++) {
                        hashMap1.put(i, "value" + i);
                        try {
                            Thread.sleep(new Random().nextInt(10));
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    } catch (Exception e) {

                    } finally {
                        //lock.unlock();
                    }
                }
            }).start();


        //读线程
        new Thread(new Runnable() {
            @Override
            public void run() {

                while (true) {
                    lock.lock();
                    try {
                        if (hashMap1.size() > 0) {
                            for (Map.Entry entry : hashMap1.entrySet()) {
                                System.out.println(entry.getKey() + "=" + entry.getValue());
                            }
                        }
                    } catch (Exception e) {
                        System.out.println(e.getMessage());
                    } finally {
                        lock.unlock();
                    }


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