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How to Use the @Cleanup Annotation of Project Lombok

This post will focus on how to make use of the @Cleanup annotation of Project Lombok. The Lombok Project is a java library that helps a developer generate boilerplate code. By simply adding the Lombok library to your IDE and build path, the Lombok library will auto-generate the Java bytecode, as per the annotations, into the .class files.

The @Cleanup annotation is added to any local variable declaration of a resource that needs to be closed after the successful execution of the resource. Project Lombok will generate the try / finally clauses and invoking the close() method.

Getting Started

The following list defines the technologies and libraries I used to implement the sample code:

Multiple Posts

This will form part of a multi-part series on how to use the Project Lombok to generate boilerplate code in java projects.

Code Example

The code example can be downloaded from Github from the button below.

Download Code

Define Maven Dependencies

The following dependencies should be included in the pom.xml file.

<dependency>
  <groupId>org.projectlombok</groupId>
  <artifactId>lombok</artifactId>
  <version>1.18.8</version>
  <scope>provided</scope>
</dependency>

Example: @Cleanup

The @Cleanup annotation ensures that resources are automatically cleaned up by adding the try .. finally blocks and invoking the close() method on the resource. This example illustrates how to add the @Cleanup annotation to a local variable and as a result, the close method will be invoked within a try/finally construct. Look at the sample code below to see how this works.

Origial Class

public class CleanupExample01 {

    public static void main(String[] args) throws IOException {
        @Cleanup InputStream in = new FileInputStream("inputFile.txt");
        @Cleanup OutputStream out = new FileOutputStream("outputFile.txt");

        byte[] b = new byte[10000];
        while (true) {
            int r = in.read(b);
            if (r == -1) break;
            out.write(b, 0, r);
        }
    }
}

To truly appreciate the magic of the Lombok library, you should compile the CleanupExample01 class by making use of maven to build your project. As part of the compile process, Lombok will generate the boilerplate code depending on the type of annotation you used. To see the boilerplate code, you should decompile CleanupExample01.class file. An easy way to do this is to make use of the following URL:

http://www.javadecompilers.com

Decompiled Class

public static void main(final String[] args) throws IOException {
    final InputStream in = new FileInputStream("inputFile.txt");
    try {
        final OutputStream out = new FileOutputStream("outputFile.txt");
        try {
            final byte[] b = new byte[10000];
            while (true) {
                final int r = in.read(b);
                if (r == -1) {
                    break;
                }
                out.write(b, 0, r);
            }
        }
        finally {
            if (Collections.singletonList(out).get(0) != null) {
                out.close();
            }
        }
    }
    finally {
        if (Collections.singletonList(in).get(0) != null) {
            in.close();
        }
    }
}

Summary

Congratulations !!! You have successfully used the Cleanup annotation of Project Lombok. Please look out for more examples on how to make use of Project Lombok to simplify your Java coding experience.