使用Java执行命令行程序时仅接收到null

Question

Okay, so I have been experimenting with the Process and Runtime classes and I have run into a problem. When I try to execute this command : cmd /c dir , the output is null. Here is a snippet of my code:

try {
    Runtime runtime = Runtime.getRuntime();
    Process process = runtime.exec("cmd /c dir");

    BufferedReader output = new BufferedReader(new InputStreamReader(process.getInputStream()));

    //BufferedReader serverOutputError = new BufferedReader(new InputStreamReader(serverStart.getErrorStream()));

    String line = null;

    while ((output.readLine()) != null) {
        System.out.println(line);
    }

    int exitValue = process.waitFor();
    System.out.println("Command exited with exit value: " + exitValue);

    process.destroy();
    System.out.println("destroyed");
} catch (IOException e) {
    e.printStackTrace();
} catch (InterruptedException e) {
    e.printStackTrace();
}

And I get this for an output:

(18 lines of just "null")
Command exited with exit value: 0
destroyed

Any ideas?

Answer 1

You never set your line variable that you are using to write to the console.

Replace

while ((output.readLine()) != null) {

with

while ((line = output.readLine()) != null) {

Answer 2

try it like this:

String line = output.readLine();

while (line != null) {
    System.out.println(line);
    line = output.readLine();
}

Answer 3

while ((output.readLine()) != null) {
    System.out.println(line);
}

should be

while ((line = output.readLine()) != null) {
    System.out.println(line);
}

Answer 4

String line = null;
while ((output.readLine()) != null) {
        System.out.println(line);
    }

Here is your problem. You never set line to anything in your loop. It remains null. You need to set line to value of output.readLine().

while((line = output.readLine()) != null)