Learning test to read the contents of a file at a specific revision with JGit , emulates git show <branch>:<file>

ObjectLoaderLearningTest.java

package com.codeaffine.jgit.example;

import static java.nio.charset.StandardCharsets.UTF_8;
import static org.junit.Assert.assertEquals;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;

import org.eclipse.jgit.api.Git;
import org.eclipse.jgit.api.errors.GitAPIException;
import org.eclipse.jgit.lib.ObjectId;
import org.eclipse.jgit.lib.ObjectLoader;
import org.eclipse.jgit.lib.ObjectReader;
import org.eclipse.jgit.lib.Repository;
import org.eclipse.jgit.treewalk.TreeWalk;
import org.junit.After;
import org.junit.Before;
import org.junit.Rule;
import org.junit.Test;
import org.junit.rules.TemporaryFolder;

public class ObjectLoaderLearningTest {

  @Rule
  public final TemporaryFolder tempFolder = new TemporaryFolder();

  private Git git;
  private Repository repository;
  
  @Before
  public void setUp() throws GitAPIException {
    git = Git.init().setDirectory( tempFolder.getRoot() ).call();
    repository = git.getRepository();
  }

  @After
  public void tearDown() {
    repository.close();
  }

  @Test
  public void testShow() throws Exception {
    String helloWorld = "Hello World!";
    writeFile( "readme", helloWorld );
    commitAll( "Add readme file" );
    ObjectId masterTreeId = git.getRepository().resolve( "refs/heads/master^{tree}" );
    
    TreeWalk treeWalk = TreeWalk.forPath( git.getRepository(), "readme", masterTreeId );
    ObjectId blobId = treeWalk.getObjectId( 0 );
    ObjectLoader objectLoader = loadObject( blobId );
    byte[] bytes = objectLoader.getBytes();
    treeWalk.close();

    assertEquals( helloWorld, new String( bytes, UTF_8 ) );
  }

  private void commitAll( String message ) throws Exception {
    git.add().addFilepattern( "." ).call();
    git.commit().setMessage( message ).call();
  }
  
  private ObjectLoader loadObject( ObjectId objectId ) throws IOException {
    ObjectReader objectReader = repository.newObjectReader();
    ObjectLoader result = objectReader.open( objectId );
    objectReader.close();
    return result;
  }

  private void writeFile( String name, String content ) throws IOException {
    File file = new File( git.getRepository().getWorkTree(), name );
    try( FileOutputStream outputStream = new FileOutputStream( file ) ) {
      outputStream.write( content.getBytes( UTF_8 ) );
    }
  }

}