如果在同步块之外声明/返回了返回值，该方法是否是线程安全的？

Question

public Foo getFoo(){
    Foo foo = null;

    synchronized(fooList){
        if(fooList.size() > 0){
            foo = fooList.remove(0);
        }
    }

    return foo;
}

Since foo is declared outside of the synchronized block, does the potential exist of returning bad data?

Answer 1

Each thread instance calling getFoo() will have its own foo instance. Thus foo is thread safe and doesn't need synchronization.

Answer 2

What does "bad data" mean in this context? fooList may change asynchronously before synchronized(fooList) and after the corresponding closing brace, before return foo; (more generally speaking, up to the moment the returned value is used.) What is your ultimate goal?

Answer 3

getFoo will not return stale data, since Foo foo is local variable AND fooList is synchronized

Local variable is thread safe since each thread call will create a new Foo object, instead of sharing single object. While instead variable is not thread safe, since multiple threads can access fooList , but in this case the fooList is already synchronized.