Make output stream optional in DiffCommand

Use the NullOutputStream.INSTANCE value when the
configured output stream is null or the command is
configured to only show name and status.

Also only set the context and prefix options if
formatting is actually being performed.

Bug: 377157
Change-Id: I333cfcc82ee746f3c6a8e94c09dcc803ffbb4b3a
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/api/DiffCommandTest.java

diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/api/DiffCommandTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/api/DiffCommandTest.java
index 16a3d60..1aa74e8 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/api/DiffCommandTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/api/DiffCommandTest.java
@@ -43,6 +43,8 @@
 package org.eclipse.jgit.api;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertTrue;
 
 import java.io.ByteArrayOutputStream;
 import java.io.File;
@@ -222,6 +224,23 @@ public void testDiffWithNegativeLineCount() throws Exception {
 		assertEquals(expected.toString(), actual);
 	}
 
+	@Test
+	public void testNoOutputStreamSet() throws Exception {
+		File file = writeTrashFile("test.txt", "a");
+		assertTrue(file.setLastModified(file.lastModified() - 5000));
+		Git git = new Git(db);
+		git.add().addFilepattern(".").call();
+		write(file, "b");
+
+		List<DiffEntry> diffs = git.diff().call();
+		assertNotNull(diffs);
+		assertEquals(1, diffs.size());
+		DiffEntry diff = diffs.get(0);
+		assertEquals(ChangeType.MODIFY, diff.getChangeType());
+		assertEquals("test.txt", diff.getOldPath());
+		assertEquals("test.txt", diff.getNewPath());
+	}
+
 	private AbstractTreeIterator getTreeIterator(String name)
 			throws IOException {
 		final ObjectId id = db.resolve(name);

org.eclipse.jgit/src/org/eclipse/jgit/api/DiffCommand.java

diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/api/DiffCommand.java b/org.eclipse.jgit/src/org/eclipse/jgit/api/DiffCommand.java
index 3a50c7a..e82b048 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/api/DiffCommand.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/api/DiffCommand.java
@@ -64,6 +64,7 @@
 import org.eclipse.jgit.treewalk.CanonicalTreeParser;
 import org.eclipse.jgit.treewalk.FileTreeIterator;
 import org.eclipse.jgit.treewalk.filter.TreeFilter;
+import org.eclipse.jgit.util.io.NullOutputStream;
 
 /**
  * Show changes between commits, commit and working tree, etc.
@@ -108,8 +109,11 @@ protected DiffCommand(Repository repo) {
 	 * @return a DiffEntry for each path which is different
 	 */
 	public List<DiffEntry> call() throws GitAPIException, IOException {
-		final DiffFormatter diffFmt = new DiffFormatter(
-				new BufferedOutputStream(out));
+		final DiffFormatter diffFmt;
+		if (out != null && !showNameAndStatusOnly)
+			diffFmt = new DiffFormatter(new BufferedOutputStream(out));
+		else
+			diffFmt = new DiffFormatter(NullOutputStream.INSTANCE);
 		diffFmt.setRepository(repo);
 		diffFmt.setProgressMonitor(monitor);
 		try {
@@ -136,17 +140,17 @@ public List<DiffEntry> call() throws GitAPIException, IOException {
 			}
 
 			diffFmt.setPathFilter(pathFilter);
-			if (contextLines >= 0)
-				diffFmt.setContext(contextLines);
-			if (destinationPrefix != null)
-				diffFmt.setNewPrefix(destinationPrefix);
-			if (sourcePrefix != null)
-				diffFmt.setOldPrefix(sourcePrefix);
 
 			List<DiffEntry> result = diffFmt.scan(oldTree, newTree);
-			if (showNameAndStatusOnly) {
+			if (showNameAndStatusOnly)
 				return result;
-			} else {
+			else {
+				if (contextLines >= 0)
+					diffFmt.setContext(contextLines);
+				if (destinationPrefix != null)
+					diffFmt.setNewPrefix(destinationPrefix);
+				if (sourcePrefix != null)
+					diffFmt.setOldPrefix(sourcePrefix);
 				diffFmt.format(result);
 				diffFmt.flush();
 				return result;