在Java中检查对象是否为空

Question

I am writing my code in Java where I am creating an object and and accessing it in two different threads. My first thread1 thread calls some public methods on this object at runtime.

final Thread thread1 = new Thread(){
    @Override
    public void run() {
        myObj.pubFunc1();
        myObj.puFunc2();
        myObj.pubFunc3();
    }
};

I have another thread thread2 which might release this object and set it to null like:

final Thread thread2 = new Thread(){
    @Override
    public void run() {
        myObj.release();
        myObj = null;
    }
};

My question is if I should put check for null around each statement in my thread1 like this?

final Thread thread1 = new Thread(){
        @Override
        public void run() {
            if(myObj != null) {
                myObj.pubFunc1();
            }
            if(myObj != null) {
                myObj.pubFunc2();
            }
            if(myObj != null) {
                myObj.pubFunc3();
            }
        }
    };

OR only one check around all the statements is enough? The basic question that might originate from this is there is a lock around the object on which we have made a null check? How to handle this situation. What design pattern should I use to handle this situation.

NOTE : I do not want the three statements to be necessarily executed and I even do no want that the three statements form an atomic unit. I just want to perform an operation if the object I am performing the operation on is not null.

Answer 1

The design your propose is flawed. Imagine this execution:

• myObj = new MyObject();
• Thread 2: if (myObjec != null) => all good, object is not null
• Thread 1: myObj.release(); myObj = null; myObj.release(); myObj = null;
• Thread 2: myObj.pubFunc1(); => Boom, NPE.

I think you only have 2 options:

• either you synchronize the accesses to myObj to make the if/pubFunc calls atomic

• or you save a local copy of the object - this may or may not be acceptable depending on your use case:

   public void run() { MyObject localObj = myObj; if (localObj != null { //NOTE: myObj may have been released localObj.pubFunc1(); localObj.puFunc2(); localObj.pubFunc3(); } } 

Note: I assume that you are aware of visibility issues and that myObj is properly published/synchronized.