java thread communication, independent file reading and wiriting

问题

Java. I have two threads. one will be continuously monitoring for some events and based on the events, it will be updating (addition or deletion) a file. the other thread which is a timer task event, will be updating the same file in regular intervals of time. I want the threads to update the file when the other thread is not accessing the file. I couldn't use locks as file updating code part is independent for each thread and in different java classes. how can i do so? thanks in advance.

回答1:

You can use synchronization.

public synchronized void update() {
..
..
..
}

so only one thread access the method. Other will wait on the lock.

If you have add,update,delete method then,

private static Object LOCK = new Object(); 

public void update() {
  synchronized(LOCK) {
    ..
    ..
    ..
    }
}

public void add() {
  synchronized(LOCK) {
    ..
    ..
    ..
    }
}

public void delete() {
  synchronized(LOCK) {
    ..
    ..
    ..
    }
}

So Only one thread can able to edit/delete/add the file.

if one thread is adding , second thread is trying to delete then it will wait first thread to add.

回答2:

synchronized  void updateFILE
{
       //Your operation here
}

So at a time one thread can perform opetation.. You can have look at here

回答3:

Perhaps you could:

1. Create a wrapper class around your unsafe file-updater-class.
2. Make that wrapper class thread-safe by adding your synchronization or locks that clearly defines the critical sections and handle all exceptions appropriately (for example, unlock the critical section in a finally-block)
3. Let the other threads call your wrapper class instead.

来源：https://stackoverflow.com/questions/27478888/java-thread-communication-independent-file-reading-and-wiriting