JGit.java

import org.eclipse.jgit.api.Git;
import org.eclipse.jgit.api.MergeCommand;
import org.eclipse.jgit.api.errors.GitAPIException;
import org.eclipse.jgit.diff.DiffEntry;
import org.eclipse.jgit.diff.DiffFormatter;
import org.eclipse.jgit.lib.ObjectId;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.IOException;
import java.util.List;

public class JGit {

    public static void main(String[] args) throws GitAPIException, IOException {
        //git clone https://github.com/orekyuu/JavaBeamStudio.git
        File test = new File("testrepo");
        Git git;
        if (test.exists()) {
            git = Git.open(test);
        } else {
            git = Git.cloneRepository()
                    .setURI("https://github.com/orekyuu/JavaBeamStudio.git")
                    .setDirectory(test)
                    .call();
        }

        //git checkout develop
        git.checkout().setName("develop").call();

        ObjectId objectId = git.getRepository().resolve("remotes/origin/css-update");
        //git merge --no-ff --no-commit css-update
        git.merge()
                .setFastForward(MergeCommand.FastForwardMode.NO_FF)
                .setCommit(false)
                .include(objectId)
                .call();

        //git diff ?
        List<DiffEntry> call = git.diff().call();

        //diffの差分が見たい
        for (DiffEntry entry : call) {
            try (ByteArrayOutputStream out = new ByteArrayOutputStream();
                 DiffFormatter formatter = new DiffFormatter(out)) {
                formatter.setRepository(git.getRepository());
                formatter.format(entry);// org.eclipse.jgit.errors.MissingObjectException: Missing blob 0ca666588a0f454e6fa94e4a86c8fdfa67986d5c
                String diffText = new String(out.toByteArray());
                System.out.println(diffText);
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

    }
}