Java transient关键字和null

Question

I'm using Intellij IDEA, and the follow code generates a warning: " Expression testObj might evaluate to null but is returned by the method declared as @NotNull (at line 16) "

package com.dlp;

import javax.annotation.Nonnull;
import javax.annotation.Nullable;

public class TestCase {
    @Nullable
    private transient Object testObj = null;

    @Nonnull
    public Object getTestObj() {
        if(testObj == null) {
            testObj = new Object();
        }

        return testObj; //Line 16
    }
}

This only happens if I mark testObj as transient. Removing the keyword clears the warning. Is there some interaction between transient and nullability that I'm not getting, or is this just a bug in IDEA?

Answer 1

Nullability is very strict in the compiler. Yes, you do check for null, but there is a problem in that your code might be used in a multi-threaded context. Consider this scenario:

This block is executed:

    if(testObj == null) {
        testObj = new Object();
    }

The current thread is pre-empted and another thread comes along and sets testObj to null. Then the original thread becomes live again and finishes execution and returns null for testObj .

The proper way to write this code to avoid the null warning is like this:

public Object getTestObj() {
    Object tmpObj = testObj
    if(tmpObj == null) {
        tmpObj = new Object();
        testObj = tmpObj;
    }

    return tmpObj;
}

In this case, you are guaranteed that the return value is never null.

Granted, this is a pretty tight corner case and most programs will never need this level of safety, but that is how compilers should be checking for null.

The fact that the null warning disappears when you add transient, seems like an IntelliJ bug. However, the suggested code above is the proper way to handle nullability for fields. If this is too onerous for you, I'd suggest avoiding adding null annotations on fields.