Process output from apache-commons exec

问题:

I am at my wits end here. I'm sure this is something simple and I most likely have huge holes in my understanding of java and streams. I think there are so many classes that I'm a bit overwhelmed with trying to poke through the API to figure out when and how I want to use the multitude of input/output streams.

I just learned about the existence of the apache commons library (self teaching java fail), and am currently trying to convert some of my Runtime.getRuntime().exec to use the commons - exec. Already it's fixed some of the once every 6 months this problem crops up then goes away style problems with exec.

The code executes a perl script, and displays the stdout from the script in the GUI as it is running.

The calling code is inside of a swingworker.

I'm getting lost how to use the pumpStreamHandler... anyway here is the old code:

String pl_cmd = "perl script.pl"
Process p_pl = Runtime.getRuntime().exec( pl_cmd );

BufferedReader br_pl = new BufferedReader( new InputStreamReader( p_pl.getInputStream() ) );

stdout = br_pl.readLine();
while ( stdout != null )
{
    output.displayln( stdout );
    stdout = br_pl.readLine();
}

I guess this is what I get for copy pasting code I don't fully understand a long time ago. The above I assume is executing the process, then grabs the outputstream (via "getInputStream"?), places it into a buffered reader, then will just loop there until the buffer is empty.

What I don't get is why there is no need for a 'waitfor' style command here? Isn't it possible that there will be some time in which the buffer will be empty, exit the loop, and continue on while the process is still going? When I run it, this doesn't seem to be the case.

In any event, I'm trying to get the same behavior using commons exec, basically again going from google found code:

DefaultExecuteResultHandler rh = new DefaultExecuteResultHandler();
ExecuteWatchdog wd  = new ExecuteWatchdog( ExecuteWatchdog.INFINITE_TIMEOUT );
Executor exec = new DefaultExecutor();

ByteArrayOutputStream out = new ByteArrayOutputStream();
PumpStreamHandler psh = new PumpStreamHandler( out );

exec.setStreamHandler( psh );
exec.setWatchdog( wd );

exec.execute(cmd, rh );
rh.waitFor();

I'm trying to figure out what pumpstreamhandler is doing. I assume that this will take the output from the exec object, and fill the OutputStream I provide it with the bytes from the perl script's stdout/err?

If so how would you get the above behavior to have it stream the output line by line? In examples people show you call the out.toString() at the end, and I assume this would just give me a dump of all the output from the script once it is done running? How would you do it such that it would show the output as it is running line by line?

------------Future Edit ---------------------

Found this via google and works nice as well:

public static void main(String a[]) throws Exception
{
    ByteArrayOutputStream stdout = new ByteArrayOutputStream();
    PumpStreamHandler psh = new PumpStreamHandler(stdout);
    CommandLine cl = CommandLine.parse("ls -al");
    DefaultExecutor exec = new DefaultExecutor();
    exec.setStreamHandler(psh);
    exec.execute(cl);
    System.out.println(stdout.toString());
}

回答1:

Don't pass a ByteArrayOutputStream to the PumpStreamHandler, use an implementation of the abstract class org.apache.commons.exec.LogOutputStream. Something like this:

import java.util.LinkedList;
import java.util.List;
import org.apache.commons.exec.LogOutputStream;

public class CollectingLogOutputStream extends LogOutputStream {
    private final List<String> lines = new LinkedList<String>();
    @Override protected void processLine(String line, int level) {
        lines.add(line);
    }   
    public List<String> getLines() {
        return lines;
    }
}

Then after the blocking call to exec.execute your getLines() will have the standard out and standard error you are looking for. The ExecutionResultHandler is optional from the perspective of just executing the process, and collecting all the stdOut/stdErr into a list of lines.

回答2:

What I don't get is why there is no need for a 'waitfor' style command here? Isn't it possible that there will be some time in which the buffer will be empty, exit the loop, and continue on while the process is still going? When I run it, this doesn't seem to be the case.

readLine blocks. That is, your code will wait until a line has been read.

PumpStreamHandler

from Documentation

Copies standard output and error of subprocesses to standard output and error of the parent process. If output or error stream are set to null, any feedback from that stream will be lost.

回答3:

Based on James A Wilson's answer I created the helper class "Execute". It wraps his answer into a solution that also supplies the exitValue for convenience.

A single line is necessary to execute a command this way:

ExecResult result=Execute.execCmd(cmd,expectedExitCode);

The following Junit Testcase tests and shows how to use it:

Junit4 test case:

package com.bitplan.newsletter;

import static org.junit.Assert.*;

import java.util.List;

import org.junit.Test;

import com.bitplan.cmd.Execute;
import com.bitplan.cmd.Execute.ExecResult;

/**
 * test case for the execute class
 * @author wf
 *
 */
public class TestExecute {
     @Test
   public void testExecute() throws Exception {
     String cmd="/bin/ls";
     ExecResult result = Execute.execCmd(cmd,0);
     assertEquals(0,result.getExitCode());
     List<String> lines = result.getLines();
     assertTrue(lines.size()>0);
     for (String line:lines) {
         System.out.println(line);
     }
   }
}

Execute Java helper Class:

package com.bitplan.cmd;

import java.util.LinkedList;
import java.util.List;
import java.util.logging.Level;

import org.apache.commons.exec.CommandLine;
import org.apache.commons.exec.DefaultExecutor;
import org.apache.commons.exec.LogOutputStream;
import org.apache.commons.exec.PumpStreamHandler;

/**
 * Execute helper using apache commons exed
 *
 *  add this dependency to your pom.xml:
   <dependency>
            <groupId>org.apache.commons</groupId>
            <artifactId>commons-exec</artifactId>
            <version>1.2</version>
        </dependency>

 * @author wf
 *
 */
public class Execute {

    protected static java.util.logging.Logger LOGGER = java.util.logging.Logger
            .getLogger("com.bitplan.cmd");

    protected final static boolean debug=true;

    /**
     * LogOutputStream
     * http://stackoverflow.com/questions/7340452/process-output-from
     * -apache-commons-exec
     * 
     * @author wf
     * 
     */
    public static class ExecResult extends LogOutputStream {
        private int exitCode;
        /**
         * @return the exitCode
         */
        public int getExitCode() {
            return exitCode;
        }

        /**
         * @param exitCode the exitCode to set
         */
        public void setExitCode(int exitCode) {
            this.exitCode = exitCode;
        }

        private final List<String> lines = new LinkedList<String>();

        @Override
        protected void processLine(String line, int level) {
            lines.add(line);
        }

        public List<String> getLines() {
            return lines;
        }
    }

    /**
     * execute the given command
     * @param cmd - the command 
     * @param exitValue - the expected exit Value
     * @return the output as lines and exit Code
     * @throws Exception
     */
    public static ExecResult execCmd(String cmd, int exitValue) throws Exception {
        if (debug)
            LOGGER.log(Level.INFO,"running "+cmd);
        CommandLine commandLine = CommandLine.parse(cmd);
        DefaultExecutor executor = new DefaultExecutor();
        executor.setExitValue(exitValue);
        ExecResult result =new ExecResult();
        executor.setStreamHandler(new PumpStreamHandler(result));
        result.setExitCode(executor.execute(commandLine));
        return result;
    }

}