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RAII in Java

java OOPoop

Resource Acquisition Is Initialization (RAII) is a design idea introduced in C++ by Bjarne Stroustrup for exception-safe resource management. Thanks to garbage collection Java doesn't have this feature, but we can implement something similar, using try-with-resources.

At Sachem Farm (1998) by John Huddles
At Sachem Farm (1998) by John Huddles

The problem RAII is solving is obvious; have a look at this code (I'm sure you know what Semaphore is and how it works in Java):

class Foo {
  private Semaphore sem = new Semaphore(5);
  void print(int x) throws Exception {
    this.sem.acquire();
    if (x > 1000) {
      throw new Exception("Too large!");
    }
    System.out.printf("x = %d", x);
    this.sem.release();
  }
}

The code is rather primitive and doesn't do anything useful, but you most probably get the idea: the method print(), if being called from multiple parallel threads, will allow only five of them to print in parallel. Sometimes it will not allow some of them to print and will throw an exception if x is bigger than 1000.

The problem with this code is—resource leakage. Each print() call with x larger than 1000 will take one permit from the semaphore and won't return it. In five calls with exceptions the semaphore will be empty and all other threads won't print anything.

What is the solution? Here it is:

class Foo {
  private Semaphore sem = new Semaphore(5);
  void print(int x) throws Exception {
    this.sem.acquire();
    if (x > 1000) {
      this.sem.release();
      throw new Exception("Too large!");
    }
    System.out.printf("x = %d", x);
    this.sem.release();
  }
}

We must release the permit before we throw the exception.

However, there is another problem that shows up: code duplication. We release the permit in two places. If we add more throw instructions we will also have to add more sem.release() calls.

A very elegant solution was introduced in C++ and is called RAII. This is how it would look in Java:

class Permit {
  private Semaphore sem;
  Permit(Semaphore s) {
    this.sem = s;
    this.sem.acquire();
  }
  @Override
  public void finalize() {
    this.sem.release();
  }
}
class Foo {
  private Semaphore sem = new Semaphore(5);
  void print(int x) throws Exception {
    new Permit(this.sem);
    if (x > 1000) {
      throw new Exception("Too large!");
    }
    System.out.printf("x = %d", x);
  }
}

See how beautiful the code is inside method Foo.print(). We just create an instance of class Permit and it immediately acquires a new permit at the semaphore. Then we exit the method print(), either by exception or in the normal way, and the method Permit.finalize() releases the permit.

Elegant, isn't it? Yes, it is, but it won't work in Java.

It won't work because, unlike C++, Java doesn't destroy objects when their scope of visibility is closed. The object of class Permit won't be destroyed when we exit the method print(). It will be destroyed eventually but we don't know when exactly. Most likely it will be destroyed way after all permits in the semaphore got acquired and we get blocked.

There is a solution in Java too. It is not as elegant as the one from C++, but it does work. Here it is:

class Permit implements Closeable {
  private Semaphore sem;
  Permit(Semaphore s) {
    this.sem = s;
  }
  @Override
  public void close() {
    this.sem.release();
  }
  public Permit acquire() {
    this.sem.acquire();
    return this;
  }
}
class Foo {
  private Semaphore sem = new Semaphore(5);
  void print(int x) throws Exception {
    try (Permit p = new Permit(this.sem).acquire()) {
      if (x > 1000) {
        throw new Exception("Too large!");
      }
      System.out.printf("x = %d", x);
    }
  }
}

Pay attention to the try block and to the Closeable interface that the class Permit now implements. The object p will be "closed" when the try block exits. It may exit either at the end, or by the return or throw statements. In either case Permit.close() will be called: this is how try-with-resources works in Java.

I introduced method acquire() and moved sem.acquire() out of the Permit constructor because I believe that constructors must be code-free.

To summarize, RAII is a perfect design pattern approach when you deal with resources that may leak. Even though Java doesn't have it out of the box we can implement it via try-with-resources and Closeable.

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