Why Doesn't Try-With-Resources Support Field Variables in Java?

Why doesn’t try-with-resources work with field variables in Java?

// Example Code for Try-With-Resources
try (BufferedReader br = new BufferedReader(new FileReader("file.txt"))) {
    // Read file
} catch (IOException e) {
    e.printStackTrace();
}

In Java, the try-with-resources statement provides a way to ensure that resources are closed automatically after their execution. However, it does not work with field variables due to specific language design considerations.

class ResourceManager {
    private BufferedReader br;

    public void readFile() throws IOException {
        try (BufferedReader br = new BufferedReader(new FileReader("file.txt"))) {
            // Read file
        } // br is closed automatically here.
    }
}

Causes

• Field variables are associated with the instance of a class, not a specific block of code, which contradicts the intention of try-with-resources.
• The try-with-resources construct is designed to only manage local variables defined within the try block.

Solutions

• Use local variables within the try block to leverage try-with-resources effectively.
• If you need to manage resources at a class level, consider using a traditional try-finally structure.

Mistake: Declaring resources as class fields and attempting to use them in try-with-resources.

Solution: Always declare resources within the try block.

Mistake: Assuming try-with-resources can manage multiple resource types declared in fields.

Solution: Use separate try statements for different resource types.