使用其他类的公共静态HashTable的同步方法。 如何使该方法线程安全？

Question

suppose I have a synchronized method that use a public static HashTable from other class. How can I make this method thread safe ? for my understanding any other thread can change this HashTable as it in other class and not in the class that holds the key (the class of the synchronized method) , doesn't it ?

Answer 1

Your understanding about the HashTable is not correct. HashTable is Synchronized and thread safe.

Look into the source code of HashTable . All the public methods are synchronized.

public synchronized V put(K key, V value) {

public synchronized V get(Object key) { 
public synchronized boolean contains(Object value) {

Answer 2

If you want to use your HashMap inside your synchronized method, it can work but only if all threads use the same instance of the class that contains both the method and the HashMap , for more clarity take a look at the example bellow :

Code Snippet

public class SynchronizedCounter {
    private int c = 0;

    public synchronized void increment() {
        c++;
    }
}

Now if all the threads use the same instance of SynchronizedCounter , we make sure only one thread is running the increment() method at time, but if each thread is instantiating his own SynchronizedCounter instance then the method won't be thread safe.

Oracle's official description here .