RAII design pattern in Java

Question

Coming from a C++ background, I am a huge fan of the RAII pattern. I have used it extensively to handle memory management and lock management along with other use cases.

With Java 1.7 I see that i can use the try-with-resources pattern to create a RAII pattern.

I created a sample application using RAII and it works, but I see compiler warnings from java.

Sample Application

try(MyResource myVar = new MyResource(..))
{
    //I am not using myVar here 
}

I get the following errors

warning: [try] auto-closeable resource node is never referenced in body of corresponding try statement

I understand the warning, it is implying that I should have used the variable inside the try block, which I don't really need to do all the time.

Looking at this I am assuming that Java doesn't really have true support for RAII and I might have misused the feature which was only for Resource Management and not exactly a RAII equivalent in C++.

Couple of questions:

1. Is my understanding correct?
2. How risky is it to ignore these warnings?
3. How do I ignore these warnings through ant?
4. Is there a simple way for me to overcome this?

for 4 i am thinking of splitting the constructor call into a simpler constructor and a instance method like this

try(MyResource myVar = new Resource())
{
   myvar.Initialize()
   ....

}

Which solves the compiler problems but takes the essence out of the RAII like design.

Jehoshaphat

So Resource has no meaningful methods (besides close()) and just causes side effects on construction? As a Java guy, that seems like an antipattern. Can you explain what the resource does? – Farika

@TomG its very similar to auto unlocking. Lock is taken on construction, unlock on destruction. But as you don't have destructors in Java, i am using auto closeable by implementing close() – Jehoshaphat

I'm still a little confused. How is this lock being shared? Creating a new Resource affects some global variable elsewhere that other code checks? – Farika

Yes exactly, there is a common shared lock. These local variables take the lock on construction and unlock on close(). wikiwand.com/en/Resource_Acquisition_Is_Initialization#/… – Jehoshaphat

Answer 1

1. Is my understanding correct?

More or less. Yes, you can use try-with-resources this way and yes, it is semantically comparable to RAII. The difference is there is no destruction or deallocation, only a method call.

It's uncommon to find objects written just to wrap some resource management logic e.g.:

import java.util.concurrent.locks.Lock;

public class Guard implements AutoCloseable {
    private final Lock lock;

    public Guard(Lock lock) {
        this.lock = lock;
        lock.lock();
    }

    @Override
    public void close() {
        lock.unlock();
    }
}

try(Guard g = new Guard(myLock)) {
    // do stuff
}

If you're working with other programmers, you might have to explain what it means to a few people but I don't personally see a problem with it if it floats your boat.

What I wouldn't recommend is writing weird code like

try(AutoCloseable a = () -> lock.unlock()) {
    lock.lock();
    // do stuff
}

which is sure to generate WTFs in code review.

2. How risky is it to ignore these warnings?

Not risky. The warning is really just a notification. You know, in case you didn't know about it.

To get rid of the warning you could try:

try(@SuppressWarnings("unused")
    MyResource myVar = new MyResource())

Or maybe see also 'How do you get *ant* to not print out javac warnings?'.

An IDE should give you the option to suppress a particular warning either globally or only for a single statement (without the annotation).

Keitt

Thank you for your answers. In the example above, the compiler would complain about Guard g being unused until i use the suppresswarnings, correct? How do i prevent littering of my code with these warnings? – Jehoshaphat

To supplement this answer, the way to ignore the warnings in ANT is like this '<compilerarg value="-Xlint:all,-try"/>' – Jehoshaphat

Maybe you could even do: try((AutoClosable)()-> lock.unlock()){ } – Silica

you need @SuppressWarnings("try"), and it has to be at the method level. See Victor Nazarov's answer, which worked for me. – Broadloom

Answer 2

To expand on Radiodef's answer. I think RAII with try-with-resources is totally acceptable pattern for java. But to actually suppress warning

• you need to use @SuppressWarnings("try") instead of @SuppressWarnings("unused").
• And add annotation on method instead of variable declaration

An example with above points applied:

   @SuppressWarnings("try")
   void myMethod1() {
       try(MyResource myVar = new MyResource(..)) {
           //I am not using myVar here 
       }
   }

Expanding on the pattern itself. I've extensively used it to manage read-write locks and it worked great.

In my code I've used the trick to sometime preemptively unlock some resource to increase concurrency, like this:

try (Guard g1 = new Guard(myLock1)) {
    someStuffThatRequiresOnlyLock1();
    try (Guard g2 = new Guard(myLock2)) {
        someStuffThatRequiresBothLocks();
        if (isSomething) {
            g1.close();
            someMoreSuffThatRequiresOnlyLock2()
        } else {
            someMoreSuffThatRequiresBothLocks();
        }
    }
}

Locks are always acquired in the same order, but unlocking is performed as needed leaving as much space for concurrent processing as possible. The adjustment to make it work with read-write locks is to modify Guard class to allow repeated closings:

public class Guard implements AutoCloseable {
    private final Lock lock;
    private boolean isClosed = false;

    public Guard(Lock lock) {
        this.lock = lock;
        lock.lock();
    }

    @Override
    public void close() {
        if (!isClosed) {
           isClosed = true;
           lock.unlock();
        }
    }
}

Update: since Java 9 you do not need to suppress any warnings for this pattern in Java. You can reference a single effectively final variable in a try-clause, this will result in no warnings:

    Guard g1 = new Guard(myLock1);
    try (g1) {
        someStuffThatRequiresOnlyLock1();
        Guard g2 = new Guard(myLock2);
        try (g2) {
            someStuffThatRequiresBothLocks();
            if (isSomething) {
                g1.close();
                someMoreSuffThatRequiresOnlyLock2()
            } else {
                someMoreSuffThatRequiresBothLocks();
            }
        }
    }

Georgettegeorgi

See stackoverflow.com/a/48145269/14731 for a solution that does not require object creation on every lock/unlock operation. The downside is that you cannot explicitly close() inside the try-block. – Eccrinology

@Gill strange, your link points to a question that was removed. Do you perhaps have an alternative link to similar information? – Quentin

Answer 3

4. Is there a simple way for me to overcome this? I am thinking of splitting the constructor call into a simpler constructor and an instance method like this:

try(MyResource myVar = new Resource()) {
   myvar.Initialize()
   ...
}

Yes, @DesertIce, I use the technique you suggested. Although I'd encourage you to use a more descriptive name like preventCompilerWarning() which is obviously a stub, instead of initialize() which looks like it does something useful.

Another, possibly better solution is:

try(MyResource myVar = new Resource())
{
   assert(myVar != null); // Prevents compiler warning about unused var.
   ...
}

Using assert() with a comment is probably better because it:

1. Is not compiled into production code.
2. Works on classes you didn't write.

I got this solution from Hemal's comment on my blog post.

Sadly, @Radiodef's solution @SuppressWarnings("unused") MyResource doesn't work (any more). @VictorNazarov's @SuppressWarnings("try") solution works, but it seems a shame to suppress an entire method that might have multiple try blocks.

I'd love to see Java implement something like Kotlin that allows you to use _ instead of a variable name whenever you're forced to declare a variable you don't use - makes what you're doing explicit (and brief). Or at least recognize variables named ignored or unused or something.

Sabrasabre

Answer 4

If it were me, I would probably refactor the logic into a utility method:

public static void withLock(Lock lock, Runnable runnable) {
    lock.lock();
    try {
        runnable.run();
    } finally {
        lock.unlock();
    }
}

And then call it like

withLock(lock, () -> {
    // ... logic here
});

Or if you wanted you could still use try-with-resources inside the utility method, and suppress the warning, but it has the benefit of not littering the codebase with [suppressed] warnings.

Proliferous

Answer 5

Since Java 9 you can use effectively final variables as resources in try-with-resources statements.

See Java 9 release notes.

So you can solve the "unused variable" warning like this:

MyResource myVar = new MyResource(..);
try (myVar) {
    // myVar may or may not be referenced here
}

---

See also this answer for the reasoning behind the original try-with-resource feature design.

Tadeas