Resource Acquisition Is Initialization

About 3 min

Intent

Ensure that resources are properly released when they are no longer needed by tying the resource management to object lifetime.

Explanation

Real-world example

In a car rental service, each car represents a resource. Using the RAII pattern, when a customer rents a car (acquires the resource), the car is marked as rented. When the customer returns the car (the object goes out of scope), the car is automatically made available for the next customer. This ensures that cars are properly managed and available without manual intervention for checking availability or returns.

In plain words

Resource Acquisition is Initialization allows for exception-safe resource handling and means that objects are able to manage themselves without other code to inform them that a clean-up is required after use.

Wikipedia says

Resource acquisition is initialization (RAII) is a programming idiom used in several object-oriented, statically typed programming languages to describe a particular language behavior. Resource allocation (or acquisition) is done during object creation (specifically initialization), by the constructor, while resource deallocation (release) is done during object destruction (specifically finalization), by the destructor.

Programmatic Example

The RAII pattern is a common idiom used in software design where the acquisition of a resource is done during object creation (initialization), and the release of the resource is done during object destruction. This pattern is particularly useful in dealing with resource leaks and is critical in writing exception-safe code in C++. In Java, RAII is achieved with try-with-resources statement and interfaces java.io.Closeable and AutoCloseable.

// This is an example of a resource class that implements the AutoCloseable interface.
// The resource is acquired in the constructor and released in the close method.

@Slf4j
public class SlidingDoor implements AutoCloseable {

  public SlidingDoor() {
    LOGGER.info("Sliding door opens."); // Resource acquisition is done here
  }

  @Override
  public void close() {
    LOGGER.info("Sliding door closes."); // Resource release is done here
  }
}

In the above code, SlidingDoor is a resource that implements the AutoCloseable interface. The resource (in this case, a door) is "acquired" in the constructor (the door is opened), and "released" in the close method (the door is closed).

// This is another example of a resource class that implements the Closeable interface.
// The resource is acquired in the constructor and released in the close method.

@Slf4j
public class TreasureChest implements Closeable {

  public TreasureChest() {
    LOGGER.info("Treasure chest opens."); // Resource acquisition is done here
  }

  @Override
  public void close() {
    LOGGER.info("Treasure chest closes."); // Resource release is done here
  }
}

Similarly, TreasureChest is another resource that implements the Closeable interface. The resource (a treasure chest) is "acquired" in the constructor (the chest is opened), and "released" in the close method (the chest is closed).

// This is an example of how to use the RAII pattern in Java using the try-with-resources statement.

@Slf4j
public class App {

  public static void main(String[] args) {

    try (var ignored = new SlidingDoor()) {
      LOGGER.info("Walking in.");
    }

    try (var ignored = new TreasureChest()) {
      LOGGER.info("Looting contents.");
    }
  }
}

In the main method of the App class, we see the RAII pattern in action. The try-with-resources statement is used to ensure that each resource is closed at the end of the statement. This is where the AutoCloseable or Closeable interfaces come into play. When the try block is exited (either normally or via an exception), the close method of the resource is automatically called, thus ensuring the resource is properly released.

Class diagram

Applicability

Use RAII when resources such as file handles, network connections, or memory need to be managed and automatically released.
Suitable in environments where deterministic resource management is crucial, such as real-time systems or applications with strict resource constraints.

Known Uses

Java try-with-resources statement: Ensures that resources are closed automatically at the end of the statement.
Database connections: Using connection pools where the connection is obtained at the beginning of a scope and released at the end.
File I/O: Automatically closing files using try-with-resources.

Consequences

Benefits:

Automatic and deterministic resource management.
Reduces the likelihood of resource leaks.
Enhances code readability and maintainability by clearly defining the scope of resource usage.

Trade-offs:

May introduce complexity in understanding object lifetimes.
Requires careful design to ensure all resources are correctly encapsulated.

Object Pool: Manages a pool of reusable objects to optimize resource allocation and performance, often used for resources that are expensive to create and manage.