The finally Block - Java Tutorial 5.0 英文版

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The finally Block

The final step in setting up an exception handler is to clean up before allowing control to be passed to a different part of the program. Do this by enclosing the cleanup code within a finally block. The finally block is optional and provides a cleanup mechanism regardless of what happens within the try block. Use the finally block to close files or to release other system resources.

The try block of the writeList method that you've been working with here opens a PrintWriter. The program should close that stream before exiting the writeList method. This poses a somewhat complicated problem because writeList's try block can exit in one of three ways.

1. The new FileWriter statement fails and throws an IOException.
2. The victor.elementAt(i) statement fails and throws an ArrayIndexOutOfBoundsException.
3. Everything succeeds and the try block exits normally.

The runtime system always executes the statements within the finally block regardless of what happens within the try block. So it's the perfect place to perform cleanup.

The following finally block for the writeList method cleans up and then closes the PrintWriter.

finally {
    if (out != null) { 
        System.out.println("Closing PrintWriter");
        out.close(); 
    } else { 
        System.out.println("PrintWriter not open");
    } 
} 

In the writeList example, you could provide for cleanup without the intervention of a finally block. For example, you could put the code to close the PrintWriter at the end of the try block and again within the exception handler for ArrayIndexOutOfBoundsException, as follows.

try {
    
    out.close();       //Don't do this; it duplicates code. 
    
} catch (FileNotFoundException e) {
    out.close();       //Don't do this; it duplicates code.
    System.err.println("Caught: FileNotFoundException: " 
                      + e.getMessage());
    throw new RuntimeException(e);
    
} catch (IOException e) {
    System.err.println("Caught IOException: " 
                      + e.getMessage());
}

However, this duplicates code, thus making the code difficult to read and error-prone should you modify it later. For example, if you add code that can throw a new type of exception to the try block, you have to remember to close the PrintWriter within the new exception handler.

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