Adds clear option to ProjectGenerate instruction

biz.aQute.bndall.tests/resources/ws-stalecheck/p5/gen/Buildpath.java

@@ -0,0 +1,9 @@
+package foo.bar;
+
+
+public class Buildpath {
+	public final static String VERSION = "${def;Bundle-Version;1.0.0}"; 
+    public final static String[] BUILDPATH = {
+        ${template;-buildpath;"${@}"}
+    };
+}

biz.aQute.bndall.tests/test/biz/aQute/bnd/project/ProjectGenerateTest.java

@@ -108,6 +108,74 @@ public void testSimpleGenerator() throws Exception {
 		}
 	}
 
+	@Test
+	@SuppressWarnings({
+		"unchecked", "rawtypes"
+	})
+	public void testSimpleGeneratorDontClearOutput() throws Exception {
+		try (Workspace ws = getWorkspace("resources/ws-stalecheck")) {
+			getRepo(ws);
+			Project project = ws.getProject("p5");
+			project.setProperty("-generate",
+				"gen/**.java;output=src-gen/;generate='javagen -o src-gen/ gen/**.java';clear=false");
+
+			File outputdir = project.getFile("src-gen");
+
+			assertThat(project.getGenerate()
+				.getOutputDirs()).contains(outputdir);
+
+			File in = project.getFile("gen/Buildpath.java");
+			in.setLastModified(System.currentTimeMillis() - 2000);
+
+			File out = project.getFile("src-gen/foo/bar/Buildpath.java");
+			assertThat(out).doesNotExist();
+
+			File existing = project.getFile("src-gen/test.txt");
+			assertThat(existing).doesNotExist();
+
+			assertThat(project.getGenerate()
+				.needsBuild()).isTrue();
+
+			project.getGenerate()
+				.generate(true);
+			assertThat(project.check()).isTrue();
+			assertThat(out).isFile();
+
+			assertThat(project.getGenerate()
+				.needsBuild()).isFalse();
+
+			// new we simulate modification of a source by adding a file
+			existing.createNewFile();
+
+			// no reason to build
+			assertThat(project.getGenerate()
+				.needsBuild()).isFalse();
+
+			// delete the generated file
+			IO.delete(out);
+			// in some rear cases the whole process seamed to be to fast and the
+			// created file had the exact same timestamp. So we make sure the
+			// source is newer
+			in.setLastModified(System.currentTimeMillis() + 1);
+
+			assertThat(out).doesNotExist();
+
+			assertThat(project.getGenerate()
+				.needsBuild()).isTrue();
+
+			project.getGenerate()
+				.generate(true);
+
+			// check that our test file is still there and does exist
+			assertThat(existing).isFile();
+			assertThat(project.check()).isTrue();
+			assertThat(out).isFile();
+
+			assertThat(project.getGenerate()
+				.needsBuild()).isFalse();
+		}
+	}
+
 	@Test
 	@SuppressWarnings({
 		"unchecked", "rawtypes"

biz.aQute.bndlib/src/aQute/bnd/build/Project.java

@@ -2444,8 +2444,7 @@ public void clean() throws Exception {
 		clean(getTargetDir(), "target");
 		clean(getSrcOutput(), "source output");
 		clean(getTestOutput(), "test output");
-		for (File output : getGenerate().getOutputDirs())
-			clean(output, "generate output " + output, false);
+		getGenerate().clean();
 
 		for (File src : getSourcePath()) {
 			IO.mkdirs(src);

biz.aQute.bndlib/src/aQute/bnd/build/ProjectGenerate.java

@@ -4,6 +4,7 @@
 import static aQute.bnd.result.Result.ok;
 
 import java.io.File;
+import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
 import java.util.Collections;
@@ -96,12 +97,27 @@ private Result<Void> prepare(String sourceWithDuplicate, GeneratorSpec st) {
 
 		File out = project.getFile(output);
 		if (out.isDirectory()) {
-			for (File f : out.listFiles()) {
-				IO.delete(f);
+			if (st.clear()
+				.orElseGet(() -> Boolean.TRUE)) {
+				for (File f : out.listFiles()) {
+					IO.delete(f);
+				}
 			}
 		} else {
 			out.mkdirs();
 		}
+
+		long latestModifiedSource = sourceFiles.stream()
+			.mapToLong(File::lastModified)
+			.max()
+			.getAsLong();
+
+		// the out folder serves as our trigger if a build is needed. Thus we
+		// use to output directory timestamp as marker we can compare against
+		// later. If clear is false a generator might decided to do nothing and
+		// could cause a build loop.
+		out.setLastModified(latestModifiedSource);
+
 		return Result.ok(null);
 	}
 
@@ -308,9 +324,15 @@ public Result<Set<File>> getInputs() {
 	}
 
 	public Set<File> getOutputDirs() {
+		return getOutputDirs(false);
+	}
+
+	private Set<File> getOutputDirs(boolean toClear) {
 		return project.instructions.generate()
 			.values()
 			.stream()
+			.filter(spec -> !toClear || spec.clear()
+				.orElse(Boolean.TRUE))
 			.map(GeneratorSpec::output)
 			.filter(Objects::nonNull)
 			.map(project::getFile)
@@ -349,11 +371,9 @@ public boolean needsBuild() {
 			if (outputFiles.isEmpty())
 				return true;
 
-			long latestModifiedTarget = outputFiles.stream()
-				.filter(File::isFile)
-				.mapToLong(File::lastModified)
-				.min()
-				.orElse(0);
+			File out = project.getFile(output);
+
+			long latestModifiedTarget = out.lastModified();
 
 			boolean staleFiles = latestModifiedSource > latestModifiedTarget;
 			if (staleFiles)
@@ -363,8 +383,12 @@ public boolean needsBuild() {
 	}
 
 	public void clean() {
-		getOutputDirs().stream()
-			.forEach(IO::delete);
+		for (File output : getOutputDirs(true))
+			try {
+				project.clean(output, "generate output " + output, false);
+			} catch (IOException e) {
+				Exceptions.duck(e);
+			}
 	}
 
 	@Override

biz.aQute.bndlib/src/aQute/bnd/help/instructions/ProjectInstructions.java

@@ -61,6 +61,9 @@ interface GeneratorSpec {
 
 		@SyntaxAnnotation(lead = "Specify a JAR version for a Main class plugin (name in generate must be a fqn class name)")
 		Optional<String> version();
+
+		@SyntaxAnnotation(lead = "Determins if the output directory needs to be cleared before the generator runs. The default is true.")
+		Optional<Boolean> clear();
 	}
 
 }

docs/_instructions/generate.md

@@ -9,17 +9,20 @@ since: 5.1
 Virtually all the work bnd is concerned about happens in generating the JAR file. The key idea is to _pull_ resources in the JAR, instead of the more traditional _push_ model of other builders. This works well, except for _generating source code_. This generating step must happen before the compiler is called, and the compiler is generally called before bnd becomes active. 
 
 This `-generate` instruction specifies the code generating steps that must be executed. Source code can be generated by _system_ commands or the bnd _external plugins_.
- 
+
 	-generate	::= 	clause ( ',' clause )*
 	clause		::= 	FILESET ';' 'output=' DIR (';' option )*
 	src		::=	FILESET
 	option		::= 	'system=' STRING
 			|   'generate=' STRING
-            |   'classpath=' PATH
-            |   'workingdir=' FILE
+	        |   'classpath=' PATH
+	        |   'workingdir=' FILE
+	        |   'clear=' BOOLEAN
 
 For each clause, the key of the clause is used to establish an Ant File Set, e.g. `foo/**.in`. This a glob expression with the exception that the double wildcard ('**') indicates to any depth of directories. The `output` attribute _must_ specify a directory. If the output must be compiled this directory must be on the bnd source path.
 
+The output directory will created if it does not exist. It will be cleared of any previous generate results before a run, except if the option `clear` is set to false. In this case the used Generator needs to deal with the remnants in the directory itself.
+
 If any file in the source is older than any file in the target (to any depth), or the target is empty, the clause is considered _stale_. If the clause is not stale, it is further ignored. If no further options are set on the clause, a warning is generated that some files are out of date. 
 
 If either a warning or error option is given, these will be executed on the project.
@@ -56,7 +59,7 @@ If this example is used, it is necessary to add a new _source folder_. In Eclips
     src=${^src},src-gen
 
 Assuming that the input files are in the `gen` directory, the following can be used to automatically generate the output files based on the input files.
- 
+
     -generate:   \
         gen/**.java; \ 
             output='src-gen/' ; \ 
@@ -128,4 +131,3 @@ If you the plugin source code is in the same workspace as the project using this
 
 You can take a look at the [JavaGen](https://github.com/bndtools/bnd/tree/master/biz.aQute.bnd.javagen) project in the bnd build to see how an actual external plugin is made.
 
-