A way to call JGit commands and stream their output.

GitCliJGitWrapper.java

import java.io.File;
import java.io.OutputStream;
import java.nio.charset.Charset;
import java.util.ArrayList;
import java.util.List;
import java.util.Optional;

import org.eclipse.jgit.lib.Repository;
import org.eclipse.jgit.pgm.TextBuiltin;
import org.eclipse.jgit.pgm.opt.CmdLineParser;
import org.eclipse.jgit.pgm.opt.SubcommandHandler;
import org.kohsuke.args4j.Argument;

import com.diffplug.common.base.Either;

public class GitCliJGitWrapper {
	/** Returns the output encoding for the given repository. */
	public static Charset getOutputEncoding(Optional<Repository> db) {
		return db.map(d -> d.getConfig().getString("i18n", null, "logOutputEncoding"))
				.map(Charset::forName).orElseGet(Charset::defaultCharset);
	}

	public static void rawExecute(String str, Either<Repository, File> repoOrFolder, OutputStream outputStream, OutputStream errorStream) throws Exception {
		// either db or gitDir will be null
		String[] args = split(str);
		if (!args[0].equalsIgnoreCase("git") || args.length < 2) { //$NON-NLS-1$
			throw new IllegalArgumentException("Expected 'git <command> [<args>]', was:" + str); //$NON-NLS-1$
		}
		String[] argv = new String[args.length - 1];
		System.arraycopy(args, 1, argv, 0, args.length - 1);

		Streaming bean = new Streaming();
		final CmdLineParser clp = new CmdLineParser(bean);
		clp.parseArgument(argv);

		final TextBuiltin cmd = bean.subcommand;
		cmd.outs = outputStream;
		cmd.errs = errorStream;
		cmd.init(repoOrFolder.asOptionalLeft().orElse(null), repoOrFolder.asOptionalRight().map(File::getAbsolutePath).orElse(null));
		try {
			cmd.execute(bean.arguments.toArray(new String[bean.arguments.size()]));
		} finally {
			if (cmd.outw != null) {
				cmd.outw.flush();
			}
			if (cmd.errw != null) {
				cmd.errw.flush();
			}
		}
	}

	/** Command line entry point for streaming the command output. */
	private static class Streaming {
		@Argument(index = 0, metaVar = "metaVar_command", required = true, handler = SubcommandHandler.class)
		private TextBuiltin subcommand;

		@Argument(index = 1, metaVar = "metaVar_arg")
		private List<String> arguments = new ArrayList<String>();
	}

	/**
	 * Split a command line into a string array.
	 *
	 * A copy of Gerrit's
	 * com.google.gerrit.sshd.CommandFactoryProvider#split(String)
	 *
	 * @param commandLine
	 *            a command line
	 * @return the array
	 */
	static String[] split(String commandLine) {
		final List<String> list = new ArrayList<String>();
		boolean inquote = false;
		boolean inDblQuote = false;
		StringBuilder r = new StringBuilder();
		for (int ip = 0; ip < commandLine.length();) {
			final char b = commandLine.charAt(ip++);
			switch (b) {
			case '\t':
			case ' ':
				if (inquote || inDblQuote) {
					r.append(b);
				} else if (r.length() > 0) {
					list.add(r.toString());
					r = new StringBuilder();
				}
				continue;
			case '\"':
				if (inquote) {
					r.append(b);
				} else {
					inDblQuote = !inDblQuote;
				}
				continue;
			case '\'':
				if (inDblQuote) {
					r.append(b);
				} else {
					inquote = !inquote;
				}
				continue;
			default:
				r.append(b);
				continue;
			}
		}
		if (r.length() > 0) {
			list.add(r.toString());
		}
		return list.toArray(new String[list.size()]);
	}
}