JSch Command Hang-Up: How to Avoid Deadlock When Reading Large Output?

JSch Command Output Hang-up: A Case Study

When executing commands through JSch, you may encounter situations where the code appears to hang while attempting to read from the input stream of the ChannelExec. To resolve this, it's crucial to ensure continuous reading from the output while monitoring the command's status.

Consider the following code snippet:

<code class="java">while ((numRead = reader.read(buf)) != -1) {
    // ...
}</code>

If the command produces a significant amount of output, it can fill the output buffer, causing the command to stall while waiting for the buffer to be consumed. This can lead to a deadlock scenario.

To address this issue, you should read both standard output and standard error continuously, monitoring the command status. This ensures that the output buffer remains empty, allowing the command to complete without interruptions.

<code class="java">while (true) {
    // Read from standard output
    while (in.available() > 0) { // Replace reader with in and err for error handling
        // ...
    }
    // Read from standard error
    while (err.available() > 0) { // Replace reader with in and err for error handling
        // ...
    }
    if (channel.isClosed()) {
        // ...
    }
    try { 
      Thread.sleep(1000);
    } catch (Exception ee) {
    }
}</code>

By continuously reading from the output and error streams and periodically checking the channel status, you can prevent hang-ups and ensure that commands complete successfully.

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