Fix stale buffer detection in Openable.getBuffer() for recreated jar files

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

@@ -1686,4 +1686,55 @@ public void run(){
 		}
 	}
 
+	/*
+	 * Ensures that stale buffers from deleted/recreated jars are properly detected and replaced.
+	 * This test simulates the scenario where a jar file is deleted and recreated with the same path,
+	 * and verifies that stale cached buffers are not reused.
+	 * See https://github.com/eclipse-jdt/eclipse.jdt.ui/issues/736
+	 */
+	public void testStaleBufferAfterJarRecreation() throws CoreException, IOException {
+		IJavaProject project = null;
+		try {
+			project = createJavaProject("TestStaleBuffer", new String[0], new String[] {"JCL18_LIB"}, "", JavaCore.VERSION_1_8);
+
+			String[] pathAndContents = new String[] {
+				"pack/age/X.java",
+				"package pack.age;\n" +
+				"public interface X {\n" +
+				"  String test();\n" +
+				"}"
+			};
+			addLibrary(project, "testlib.jar", "testlibsrc.zip", pathAndContents, JavaCore.VERSION_1_8);
+
+			IPackageFragmentRoot root = project.getPackageFragmentRoot(project.getProject().getFile("testlib.jar"));
+			IOrdinaryClassFile classFile1 = root.getPackageFragment("pack.age").getOrdinaryClassFile("X.class");
+			String source1 = classFile1.getSource();
+			assertNotNull("Source should be available for first jar", source1);
+			assertTrue("Source should contain 'test()' method", source1.contains("test()"));
+			assertFalse("Source should not contain 'newMethod()'", source1.contains("newMethod()"));
+
+			removeLibrary(project, "testlib.jar", "testlibsrc.zip");
+			String[] newPathAndContents = new String[] {
+				"pack/age/X.java",
+				"package pack.age;\n" +
+				"public interface X {\n" +
+				"  String newMethod();\n" +
+				"}"
+			};
+			addLibrary(project, "testlib.jar", "testlibsrc.zip", newPathAndContents, JavaCore.VERSION_1_8);
+
+			root = project.getPackageFragmentRoot(project.getProject().getFile("testlib.jar"));
+			IOrdinaryClassFile classFile2 = root.getPackageFragment("pack.age").getOrdinaryClassFile("X.class");
+			String source2 = classFile2.getSource();
+
+			assertNotNull("Source should be available for recreated jar", source2);
+			assertTrue("Source should contain new 'newMethod()'", source2.contains("newMethod()"));
+			assertFalse("Source should not contain old 'test()' method", source2.contains("test()"));
+		} finally {
+			if (project != null) {
+				deleteProject(project);
+			}
+		}
+	}
+
 }

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

@@ -454,6 +454,11 @@ protected IBuffer openBuffer(IProgressMonitor pm, IElementInfo info) throws Java
 	// Check the cache for the top-level type first
 	IType outerMostEnclosingType = getOuterMostEnclosingType();
 	IBuffer buffer = getBufferManager().getBuffer(outerMostEnclosingType.getClassFile());
+	// Validate the cached buffer is still valid (not stale from a deleted/recreated jar)
+	if (buffer != null && !(buffer instanceof NullBuffer) && buffer.isClosed()) {
+		getBufferManager().removeBuffer(buffer);
+		buffer = null;
+	}
 	if (buffer == null) {
 		SourceMapper mapper = getSourceMapper();
 		IBinaryType typeInfo = info instanceof IBinaryType ? (IBinaryType) info : null;

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

@@ -279,6 +279,11 @@ public IBuffer getBuffer() throws JavaModelException {
 		// ensure element is open
 		IElementInfo info = getElementInfo();
 		IBuffer buffer = getBufferManager().getBuffer(this);
+		if (buffer != null && !(buffer instanceof NullBuffer) && buffer.isClosed()) {
+			// stale buffer from deleted/recreated jar - evict and recreate
+			getBufferManager().removeBuffer(buffer);
+			buffer = null;
+		}
 		if (buffer == null) {
 			// try to (re)open a buffer
 			buffer = openBuffer(null, info);