Why is my JSch command hanging while reading output?

Understanding JSch Command Output Reading Issues

In your provided Java code using JSch, you encounter a situation where the program hangs while trying to read from a reader stream used to capture the command output. This article explains the cause of this issue and provides a solution.

Problem Analysis

The fundamental issue arises from not continuously consuming the output produced by the executed command. JSch commands accumulate output in an internal buffer, and if the buffer fills up before being consumed, it can cause a deadlock, where the command hangs waiting for the buffer to be cleared.

Solution

To resolve this, you must continuously read both the standard output and standard error streams of the command, even while waiting for the command to complete. This ensures that the output buffer never fills up and the command doesn't hang.

Example Code

Here's an extended version of your code that incorporates this solution:

<code class="java">ChannelExec channel = (ChannelExec)session.openChannel("exec");

channel.setCommand(...); // Set the command to execute

ByteArrayOutputStream outputBuffer = new ByteArrayOutputStream();
ByteArrayOutputStream errorBuffer = new ByteArrayOutputStream();

InputStream in = channel.getInputStream();
InputStream err = channel.getExtInputStream();

channel.connect();

byte[] tmp = new byte[1024];
while (true) {
    while (in.available() > 0) {
        int i = in.read(tmp, 0, 1024);
        if (i < 0) break;
        outputBuffer.write(tmp, 0, i);
    }
    while (err.available() > 0) {
        int i = err.read(tmp, 0, 1024);
        if (i < 0) break;
        errorBuffer.write(tmp, 0, i);
    }
    if (channel.isClosed()) {
        if ((in.available() > 0) || (err.available() > 0)) continue; 
        System.out.println("exit-status: " + channel.getExitStatus());
        break;
    }
    try { 
      Thread.sleep(1000);
    } catch (Exception ee) {
    }
}

System.out.println("output: " + outputBuffer.toString("UTF-8"));
System.out.println("error: " + errorBuffer.toString("UTF-8"));

channel.disconnect();</code>

In this example, the while loops continuously read the output streams (in and err) while monitoring the status of the command (channel.isClosed()). This ensures that all output is captured, even if the command produces a large amount of data.

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