Java 如何同步缓存读写操作

Question

我正在努力在我的应用程序中使用 ArrayList 实现一个简单的缓存。

我想同步缓存更新操作，同时更新缓存我不应该允许执行读取操作。所以一旦缓存更新完成，那么只有缓存应该允许读取。

ContextManager.java

public class ContextManager{
    private List<String> trashCanIds;
    public List<String> getIds() {
        return ids;
    }

    public void setIds(List<String> ids) {
        this.ids = ids;
    }
}

ConfigManager.java

public class ConfigManager{
    ContextManager ctxManager = new ContextManager();
    public synchronized List<String> loadIds() throws Exception {
        Utils utils = new Utils();
        List<String> listIds = null;
        String[] ids = utils.fetchIds();    
        if(Objects.nonNull(ids) && ids.length > 0) {
            listIds = new ArrayList<>(Arrays.asList(ids[0].split(",")));
        }
        ctxManager.setIds(idsList);
        return idsList;
    }
}

DeleteManager.java

public class DeleteManager {
    ConfigManager configManager = new ConfigManager();
    configManager.loadIds();
}

TestManager.java

public class TestManager {
    ContextManager contextManager = new ContextManager();
    contextManager.getIds();
}

在这段代码中，我同步了 loadIds() 方法。

需要帮助，如何防止在 loadIds() 进行时读取 getIds()。

Answer 1

您可以使用通过 ReentrantReadWriteLock 实例实现的 ReadWriteLock 接口来实现此目的。这个class可以代表你在执行getIds和loadIds操作时获取相应的锁的读写情况。事实上，

A ReadWriteLock maintains a pair of associated locks, one for read-only operations and one for writing. The read lock may be held simultaneously by multiple reader threads, so long as there are no writers. The write lock is exclusive.

https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/locks/ReadWriteLock.html

基本上，您的 loadIds 应该先获取 write-lock 才能继续其操作。如果成功，它会立即获取锁并继续计算；否则它会阻塞相应的线程，直到获得锁或抛出 InterruptedException。

另一方面，getIds 方法应该获取 read-lock。如果可用，当前线程立即获得锁的位置；否则它会阻塞相应的线程，直到获得锁或抛出 InterruptedException。

ContextManager.java

public class ContextManager{
    private List<String> trashCanIds;
    private ReadWriteLock lock;
    private Lock readLock;
    private Lock writeLock;

    public ContextManager(){
        lock = new ReentrantReadWriteLock(true);
        readLock = lock.readLock();
        writeLock = lock.writeLock();    
    }

    public List<String> getIds() {
        readLock.lock();
        try {
            List<String> tempTrashCanIds = new ArrayList(trashCanIds);
        } finally {
            readLock.unlock();
        }
        return tempTrashCanIds;
    }

    public void setIds(List<String> ids) {
        this.ids = ids;
    }

    public void readLock(){
        this.readLock.lock();
    }

    public void readUnlock(){
        this.readLock.unlock();
    }

    public void writeLock(){
        this.writeLock.lock();
    }

    public void writeUnlock(){
        this.writeLock.unlock();
    }
}

ConfigManager.java

public class ConfigManager{
    ContextManager ctxManager = new ContextManager();
    public List<String> loadIds() throws Exception {
        Utils utils = new Utils();
        List<String> listIds = null;
        String[] ids = utils.fetchIds();    
        if(Objects.nonNull(ids) && ids.length > 0) {
            listIds = new ArrayList<>(Arrays.asList(ids[0].split(",")));
        }

        ctxManager.writeLock();
        try {
            ctxManager.setIds(idsList);
        } finally {
            ctxManager.writeUnlock();
        }
        return idsList;
    }
}

Java 如何同步缓存读写操作

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