Perform a full build if output folder was deleted externally

If the output folder is deleted externally, Java builder doesn't rebuild
anything on next auto / incremental build request.

The change checks the missing output folder on the next build request
and converts build kind to a FULL_BUILD if at least one output folder in
the project is missing.

Fixes eclipse-jdt#4745

Author: iloveeclipse

org.eclipse.jdt.core/model/org/eclipse/jdt/internal/core/builder/JavaBuilder.java

@@ -28,6 +28,7 @@
 import java.util.Map.Entry;
 import java.util.Set;
 import org.eclipse.core.resources.ICommand;
+import org.eclipse.core.resources.IContainer;
 import org.eclipse.core.resources.IMarker;
 import org.eclipse.core.resources.IProject;
 import org.eclipse.core.resources.IResource;
@@ -42,6 +43,7 @@
 import org.eclipse.jdt.core.IClasspathEntry;
 import org.eclipse.jdt.core.IJavaModelMarker;
 import org.eclipse.jdt.core.IJavaModelStatusConstants;
+import org.eclipse.jdt.core.IJavaProject;
 import org.eclipse.jdt.core.JavaCore;
 import org.eclipse.jdt.core.JavaModelException;
 import org.eclipse.jdt.core.compiler.CategorizedProblem;
@@ -686,6 +688,11 @@ private int initializeBuilder(int kind, boolean forBuild) throws CoreException {
 			builtProjects = new LinkedHashSet<>();
 		}
 		builtProjects.add(projectName);
+
+		if (kind != CLEAN_BUILD) {
+			// check if we need to switch to full build due to missing output folder(s)
+			kind = checkOutputFolders(this.javaProject, kind);
+		}
 	}
 
 	this.binaryLocationsPerProject = new HashMap<>(3);
@@ -878,6 +885,55 @@ private void recordNewState(State state) {
 	JavaModelManager.getJavaModelManager().setLastBuiltState(this.currentProject, state);
 }
 
+/**
+ * Checks whether all output folders for the given project exist on disk.
+ *
+ * @param project
+ *                    The Java project to check
+ * @return If any output folder is missing, {@link IncrementalProjectBuilder#FULL_BUILD} is returned and original
+ *         <code>buildKind</code> argument otherwise.
+ */
+protected int checkOutputFolders(IJavaProject project, int buildKind) {
+	try {
+		IWorkspaceRoot root = project.getProject().getWorkspace().getRoot();
+		IContainer defaultOutputLocation = root.getFolder(project.getOutputLocation());
+		boolean needsFullBuild = !existsOnDisk(defaultOutputLocation);
+		if (needsFullBuild) {
+			return FULL_BUILD;
+		}
+
+		IClasspathEntry[] classpath = project.getRawClasspath();
+		for (IClasspathEntry entry : classpath) {
+			if (entry.getEntryKind() == IClasspathEntry.CPE_SOURCE) {
+				IPath outputLocation = entry.getOutputLocation();
+				if (outputLocation != null) {
+					IContainer outputFolder = root.getFolder(outputLocation);
+					if (!existsOnDisk(outputFolder)) {
+						needsFullBuild = true;
+						break;
+					}
+				}
+			}
+		}
+
+		return needsFullBuild ? FULL_BUILD : buildKind;
+	} catch (JavaModelException e) {
+		// If we can't read the classpath, just return the original build kind
+		return buildKind;
+	}
+}
+
+private boolean existsOnDisk(IContainer defaultOutputLocation) {
+	boolean exists = defaultOutputLocation.exists();
+	// Resource is not present in workspace model
+	if (!exists) {
+		return false;
+	}
+	// check that the folder *really* exists on disk and is a folder
+	IPath location = defaultOutputLocation.getLocation();
+	return location != null && location.toFile().isDirectory();
+}
+
 /**
  * String representation for debugging purposes
  */