Try-With-Resources

In older versions of Java, managing resources that require explicit closing (such as files, database connections, or network streams) required a mandatory finally block to ensure they were closed. This led to verbose and leak-prone code.

Java 7 introduced the try-with-resources statement, which guarantees the automatic closure of all resources declared within the try block, provided they implement the AutoCloseable interface.

The Syntax

Resources are declared and initialized inside parentheses immediately after the try keyword.

try (BufferedReader br = new BufferedReader(new FileReader("file.txt"))) {
    String line = br.readLine();
    System.out.println(line);
} catch (IOException e) {
    System.out.println("I/O Error: " + e.getMessage());
}
// br is closed automatically here, even if an exception occurs

Declaring Multiple Resources

You can instantiate multiple resources inside the same try block, separating them with a semicolon ;. The resources will be closed in reverse order of their declaration.

try (
    FileReader fr = new FileReader("input.txt");
    FileWriter fw = new FileWriter("output.txt")
) {
    // Use resources
} catch (IOException e) {
    System.out.println("Error: " + e.getMessage());
}

Creating Custom Resources

A custom resource can be used in a try-with-resources block as long as it implements java.lang.AutoCloseable and overrides the close() method.

public class DatabaseConnection implements AutoCloseable {
    public void query(String sql) {
        System.out.println("Running query: " + sql);
    }

    @Override
    public void close() {
        System.out.println("Connection closed!");
    }
}

Try it yourself