[performance] determineIfOnClasspath with last project used eclipse-j…

…dt#3470

Instead of randomly looping over all projects try the last successful
first.

Tested by JavaModelTests.testCreatePkgHandleInDifferentProject()

eclipse-jdt#3470

org.eclipse.jdt.core.tests.model/src/org/eclipse/jdt/core/tests/model/JavaModelTests.java

@@ -317,6 +317,13 @@ public void testCreatePkgHandleInDifferentProject() throws CoreException {
 			"x.y [in /P1/lib [in P2]]",
 			element
 		);
+		IFolder folder2 = createFolder("/P1/lib/x/z");
+		IJavaElement element2 = JavaCore.create(folder2);
+		assertElementEquals(
+			"Unexpected element",
+			"x.z [in /P1/lib [in P2]]",
+			element2
+		);
 	} finally {
 		deleteProjects(new String[] {"P1", "P2"});
 	}

org.eclipse.jdt.core/model/org/eclipse/jdt/internal/core/JavaModelManager.java

@@ -979,7 +979,7 @@ public static IJavaElement create(IFile file, IJavaProject project) {
 		}
 		return null;
 	}
-
+	private static volatile IJavaProject lastProjectUsed;
 	/**
 	 * Returns the package fragment or package fragment root corresponding to the given folder,
 	 * its parent or great parent being the given project.
@@ -999,6 +999,14 @@ public static IJavaElement create(IFolder folder, IJavaProject project) {
 			project = JavaCore.create(folder.getProject());
 			element = determineIfOnClasspath(folder, project);
 			if (element == null) {
+				IJavaProject lastProject = lastProjectUsed;
+				if (lastProject != null) {
+					// try to avoid searching through all projects
+					element = determineIfOnClasspath(folder, lastProject);
+					if (element != null) {
+						return element;
+					}
+				}
 				// walk all projects and find one that have the given folder on its classpath
 				IJavaProject[] projects;
 				try {
@@ -1007,10 +1015,13 @@ public static IJavaElement create(IFolder folder, IJavaProject project) {
 					return null;
 				}
 				for (IJavaProject p : projects) {
-					project = p;
-					element = determineIfOnClasspath(folder, project);
-					if (element != null)
-						break;
+					if (!p.equals(lastProject)) {
+						element = determineIfOnClasspath(folder, p);
+						if (element != null) {
+							lastProjectUsed = p;
+							return element;
+						}
+					}
 				}
 			}
 		} else {