java - 为什么这个线程（WriteData）没有做它的功能？

Question

我正在练习 Semaphore 类，我创建了两个线程，其中一个将数据写入“Shared”类中的 HashMap，另一个从 Shared 类中读取数据。

所以我的问题是当我读取数据时，它只显示之前添加到共享类中的数据，而不是更新的数据。我不知道我在哪里失踪。

如果是，我应该从“Writedata”类中读取线程如何返回 HashMap。

class Shared {

    Map<String, Integer> hashmap = new HashMap<String, Integer>();
    {
        hashmap.put("kishore", 797979);
        hashmap.put("nanesg", 8768787);
        hashmap.put("mahesh", 842803);
        hashmap.put("srikar", 7979980);
    }

    public void puthash(String name, Integer in, Map<String, Integer> hashmap) {
        hashmap.put(name, in);
    }

    public Map<String, Integer> gethash() {
        return this.hashmap;
    }

}

class ReadHashMap implements Runnable {

    Semaphore sem;
    String name;
    Shared shared = new Shared();

    ReadHashMap(Semaphore sem, String name) {
        this.sem = sem;
        this.name = name;

        new Thread(this, name).start();

    }

    public void run() {
        try {
            System.out.println(Thread.currentThread().getName() + " Thread trying to acquiring the permit ");
            sem.acquire();
            System.out.println(Thread.currentThread().getName() + " Thread acquires the permit ");
            Map<String, Integer> hashmap = shared.gethash();
            Set<Entry<String, Integer>> set = hashmap.entrySet();
            System.out.println("reading data in hashmap, Here is the data: ");
            for (Map.Entry<String, Integer> mp : set) {

                System.out.println(mp.getKey() + "  : " + mp.getValue());
            }

        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } finally {
            System.out.println(Thread.currentThread().getName() + " Thread relesing the permit ");
            sem.release();
        }

    }
}

class WriteData implements Runnable {

    Semaphore sem;
    String name;
    Shared shared = new Shared();

    WriteData(Semaphore sem, String name) {
        this.sem = sem;
        this.name = name;
        new Thread(this, name).start();

    }

    public void run() {
        try {
            System.out.println(Thread.currentThread().getName() + "Thread trying to get permit");
            sem.acquire();
            Map<String, Integer> hashmap = shared.gethash();
            System.out.println(Thread.currentThread().getName() + " Thread acquires the permit ");
            shared.puthash("added1", 98799, hashmap);
            shared.puthash("added2", 987989, hashmap);
            shared.puthash("added3", 98979, hashmap);

        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName() + " Thread relesing the permit ");
        sem.release();
    }
}

public class SemaphoreDemo {
    public static void main(String args[]) {
        Semaphore sem = new Semaphore(1);

        new WriteData(sem, "write");
        new ReadHashMap(sem, "read");
    }
}

这是它的输出：

writeThread trying to get permit
read Thread trying to acquiring the permit 
write Thread acquires the permit 
write Thread relesing the permit 
read Thread acquires the permit 
reading data in hashmap, Here is the data: 
nanesg  : 8768787
mahesh  : 842803
kishore  : 797979
srikar  : 7979980
read Thread relesing the permit 

为什么 WriteData 线程（特别是函数）没有添加数据。该功能有什么问题？

标签： javamultithreadingsemaphore

Answer 1

因为您在和中创建了不同Shared的实例。ReadHashMapWriteData

解决方案

使用相同的Shared：

class ReadHashMap implements Runnable {

    Semaphore sem;
    String name;
    Shared shared;

    ReadHashMap(Semaphore sem, String name, Shared shared) {
        this.sem = sem;
        this.name = name;
        this.shared = shared;
        new Thread(this, name).start();
    }
    ....
}

class WriteData implements Runnable {

    Semaphore sem;
    String name;
    Shared shared;

    WriteData(Semaphore sem, String name, Shared shared) {
        this.sem = sem;
        this.name = name;
        this.shared = shared;
        new Thread(this, name).start();
    }

    ....
}

public class SemaphoreDemo {
    public static void main(String args[]) {
        Semaphore sem = new Semaphore(1);
        Shared shared = new Shared();

        new WriteData(sem, "write", shared);
        new ReadHashMap(sem, "read", shared);
    }
}

结果

writeThread trying to get permit
write Thread acquires the permit 
read Thread trying to acquiring the permit 
write Thread relesing the permit 
read Thread acquires the permit 
reading data in hashmap, Here is the data: 
nanesg  : 8768787
mahesh  : 842803
kishore  : 797979
added2  : 987989
added3  : 98979
srikar  : 7979980
added1  : 98799
read Thread relesing the permit