关闭在if语句中已初始化的PrintWriter对象

Question

This is probably a pretty basic question, but I am trying to make a Java method that executes a process and reads the inputstream created from that process and writes to a new file. I am using Runtime.getRuntime().exec(cmd[]) and I want to use different arguments for a specific index eg cmd[3] = "something" or cmd[3] = "something else". I am using a for loop to do this, but for the first index in my array of arguments for cmd[3] I want it to print to a new file with PrintWriter so I initialized it in an if statement after initializing it at the top of the for loop. It'll probably be easier if I just show the code so here you go:

public static synchronized void myMethod(String [] argument3, File file) throws IOException{

    String[] cmd = {"command", "argument 1", "argument 2"};
    PrintWriter fileWriter = null;
    Scanner fileScanner = null;
    int i;
    for(i = 0; i < argument3.length; i++){
        fileWriter = new PrintWriter(new OutputStreamWriter(new FileOutputStream(file,true)));;
        cmd[3] = argument3[i];
        if(i == 0){
            fileWriter = new PrintWriter(new OutputStreamWriter(new FileOutputStream(file,false)));
        }
        Process p = Runtime.getRuntime().exec(cmd);
        fileScanner = new Scanner(p.getInputStream());

        while(fileScanner.hasNextLine()){
            fileWriter.println(fileScanner.nextLine());
        }
        fileWriter.close();
        fileScanner.close();

    }

}

All of this works fine, but in eclipse I get a warning saying I have a resource leak where I initialize my PrintWriter in the if statement so I am just sort of curious as to why that is since I close them at the bottom of the for loop? I know it doesn't make too much of a difference since it works anyway I just want to try to cleanup my code properly. Thanks in advance

Answer 1

The important thing to note is the

throws IOException

This can be thrown and if it is thrown, your resource will not be closed. You should make sure that your resources get closed in ANY case, even if an exception is thrown. Use...

try { ... } finally {   ... close here... } 

...for that. Imagine something bad happens, while your file is read. The IOException is thrown and your close() statement is never called, which would be the dreaded resource leak mentioned by eclipse.

Answer 2

I get a warning saying I have a resource leak where I initialize my PrintWriter in the if statement.

Here's the problem:

fileWriter = new PrintWriter(new OutputStreamWriter(new FileOutputStream(file,true)));
if (i == 0) {
    fileWriter = new PrintWriter(new OutputStreamWriter(new FileOutputStream(file,false)));
}
fileWriter.close();

The first time through the loop, when that condition holds, you're creating two PrintWriters , only the second of which is closed. You're simply discarding the first one. Instead, you could initialize it like this:

fileWriter = new PrintWriter(new OutputStreamWriter(new FileOutputStream(file, i != 0)));

But that raises the question, why are you recreating the PrintWriter each time through the loop? Just create it once, outside of the loop.

The other problem is that none of your cleanup code is actually executed if an exception is thrown. What you need is a try-catch-finally block, where you can guarantee that your cleanup code is executed. If you're using at least Java 7, you can use try-with-resources , which will take care of all the cleanup for you automatically:

void myMethod(String [] argument3, File file) throws IOException {
    String[] cmd = {"command", "argument 1", "argument 2"};
    try (PrintWriter fileWriter = new PrintWriter (file)) {
        for (int i = 0; i < argument3.length; i++) {
            Process process = Runtime.getRuntime().exec(cmd);
            try (Scanner fileScanner = new Scanner(process.getInputStream())) {
                cmd[3] = argument3[i];
                while (fileScanner.hasNextLine()) {
                    fileWriter.println(fileScanner.nextLine());
                }
            }
        }
    }
}

Answer 3

Basically, VM is complaning because the two inner objects are never closed.

 for(i = 0; i < argument3.length; i++){
        fileWriter = new PrintWriter(new OutputStreamWriter(new FileOutputStream(file,true)));;
        cmd[3] = argument3[i];
        if(i == 0){
            fileWriter = new PrintWriter(new OutputStreamWriter(new FileOutputStream(file,false)));
        }
        Process p = Runtime.getRuntime().exec(cmd);
        fileScanner = new Scanner(p.getInputStream());

        while(fileScanner.hasNextLine()){
            fileWriter.println(fileScanner.nextLine());
        }
        fileWriter.close();
        fileScanner.close();

    }

You should basically separate the element definition like this:

for (i = 0; i < argument3.length; i++) {
FileOutputStream fos;    
    if (i == 0)
        fos = new FileOutputStream(file, false);
    else
        fos = new FileOutputStream(file, true);
    OutputStreamWriter osw = new OutputStreamWriter(osw);
    fileWriter = new PrintWriter(osw);
    // do the rest
    // and then...
    fileWriter.close();
    fileScanner.close();
    osw.close();
    fos.close();
}

That will remove the warning.

PS: As the previous answer suggests, moving .close methods to a finally block would be considered the best.

Answer 4

Use try catch finally block and close your Scanner and PrintWriter in finally block since finally always executes:

public static synchronized void myMethod(String[] argument3, File file) {
        String[] cmd = { "command", "argument 1", "argument 2" };
        PrintWriter fileWriter = null;
        Scanner fileScanner = null;
        try {
            for (int i = 0; i < argument3.length; i++) {
                fileWriter = new PrintWriter(new OutputStreamWriter(
                        new FileOutputStream(file, true)));
                cmd[3] = argument3[i];
                if (i == 0) {
                    fileWriter = new PrintWriter(new OutputStreamWriter(
                            new FileOutputStream(file, false)));
                }
                Process p = Runtime.getRuntime().exec(cmd);
                fileScanner = new Scanner(p.getInputStream());

                while (fileScanner.hasNextLine()) {
                    fileWriter.println(fileScanner.nextLine());
                }

            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            fileWriter.close();
            fileScanner.close();
        }
    }