8338675: javac shouldn't silently change .jar files on the classpath

src/jdk.compiler/share/classes/com/sun/tools/javac/file/JavacFileManager.java

@@ -913,13 +913,21 @@ private JavaFileObject getFileForOutput(Location location,
             if (getClassOutDir() != null) {
                 dir = getClassOutDir();
             } else {
+                // Sibling is the associated source of the class file (e.g. x/y/Foo.java).
+                // The base name for class output is the class file name (e.g. "Foo.class").
                 String baseName = fileName.basename();
-                if (sibling != null && sibling instanceof PathFileObject pathFileObject) {
+                // Use the sibling to determine the output location where possible, unless
+                // it is in a JAR/ZIP file (we don't attempt to write class files back into
+                // archives).
+                if (sibling instanceof PathFileObject pathFileObject && !pathFileObject.isJarFile()) {
                     return pathFileObject.getSibling(baseName);
                 } else {
-                    Path p = getPath(baseName);
-                    Path real = fsInfo.getCanonicalFile(p);
-                    return PathFileObject.forSimplePath(this, real, p);
+                    // Without the sibling present, we just create an output path in the
+                    // current working directory (this isn't great, but it is what older
+                    // versions of the JDK did).
+                    Path userPath = getPath(baseName);
+                    Path realPath = fsInfo.getCanonicalFile(userPath);
+                    return PathFileObject.forSimplePath(this, realPath, userPath);
                 }
             }
         } else if (location == SOURCE_OUTPUT) {

src/jdk.compiler/share/classes/com/sun/tools/javac/file/PathFileObject.java

@@ -354,6 +354,13 @@ protected PathFileObject(BaseFileManager fileManager, Path path) {
      */
     abstract PathFileObject getSibling(String basename);
 
+    /**
+     * Returns whether this file object represents a file in a JAR archive.
+     */
+    boolean isJarFile() {
+        return this instanceof JarFileObject;
+    }
+
     /**
      * Return the Path for this object.
      * @return the Path for this object.

test/langtools/tools/javac/processing/filer/TestNoOverwriteJarFiles.java

@@ -0,0 +1,310 @@
+/*
+ * Copyright (c) 2025, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @bug 8338675
+ * @summary javac shouldn't silently change .jar files on the classpath
+ * @library /tools/lib /tools/javac/lib
+ * @modules jdk.compiler/com.sun.tools.javac.api
+ *          jdk.compiler/com.sun.tools.javac.main
+ * @build toolbox.ToolBox toolbox.JarTask toolbox.JavacTask
+ * @run junit TestNoOverwriteJarFiles
+ */
+
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.parallel.Execution;
+import toolbox.JavacTask;
+import toolbox.ToolBox;
+
+import javax.annotation.processing.RoundEnvironment;
+import javax.lang.model.element.TypeElement;
+import javax.tools.FileObject;
+import java.io.IOException;
+import java.io.OutputStream;
+import java.net.URI;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.attribute.FileTime;
+import java.time.Instant;
+import java.util.Set;
+import java.util.jar.JarOutputStream;
+import java.util.zip.ZipEntry;
+
+import static javax.tools.StandardLocation.CLASS_OUTPUT;
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.junit.jupiter.api.parallel.ExecutionMode.SAME_THREAD;
+
+/**
+ * Tests that javac cannot unexpectedly modify contents of JAR files on the
+ * class path.
+ * <p>
+ * Consider the following javac behaviours:
+ * <ol>
+ * <li>If there is no source path, javac searches the classpath for sources,
+ * including inside JAR files.
+ * <li>If a Java source file was modified more recently that an existing class
+ * file, or if no class file exists, javac will compile it from the source.
+ * <li>If there is no output directory specified, javac will put compiled class
+ * files next to their corresponding sources.
+ * </ol>
+ * Taken together, this suggests that a newly compiled class file should be
+ * written back into the JAR in which its source was found, possibly overwriting
+ * an existing class file entry. This would be very problematic.
+ * <p>
+ * This test ensures javac will not modify JAR files on the classpath, even if
+ * it compiles sources contained within them. Instead, the class file will be
+ * written into the current working directory, which mimics the JDK 8 behavior.
+ *
+ * <h2>Important</h2>
+ *
+ * This test creates files from Java compilation and annotation processing, and
+ * relies on files being written to the current working directory. Since jtreg
+ * currently offers no way to run each test case in its own directory, or clean
+ * the test directory between test cases, we must be careful to:
+ * <ul>
+ *     <li>Use {@code @Execution(SAME_THREAD)} to run test cases sequentially.
+ *     <li>Clean up the test directory ourselves between test cases (via
+ *     {@code @BeforeEach}).
+ * </ul>
+ * The alternative approach would be to compile the test classes in a specified
+ * working directory unique to each test case, but this is currently only
+ * possible using a subprocess via {@code Task.Mode.EXEC} , and this has two
+ * serious disadvantages:
+ * <ul>
+ *     <li>It significantly complicates compilation setup.
+ *     <li>It prevents step-through debugging of the annotation processor.
+ * </ul>
+ */
+@Execution(SAME_THREAD)
+public class TestNoOverwriteJarFiles {
+    private static final String LIB_SOURCE_FILE_NAME = "lib/LibClass.java";
+    private static final String LIB_CLASS_FILE_NAME = "lib/LibClass.class";
+    private static final String LIB_CLASS_TYPE_NAME = "lib.LibClass";
+
+    private static final Path TEST_LIB_JAR = Path.of("lib.jar");
+    private static final Path OUTPUT_CLASS_FILE = Path.of("LibClass.class");
+
+    // Source which can only compile against the Java source in the test library.
+    public static final String TARGET_SOURCE =
+            """
+            class TargetClass {
+                static final String VALUE = lib.LibClass.NEW_FIELD;
+            }
+            """;
+
+    // Not expensive to create, but conceptually a singleton.
+    private static final ToolBox toolBox = new ToolBox();
+
+    @BeforeAll
+    public static void ensureEmptyTestDirectory() throws IOException {
+        try (var files = Files.walk(Path.of("."), 1)) {
+            // Always includes the given path as the first returned element, so skip it.
+            if (files.skip(1).findFirst().isPresent()) {
+                throw new IllegalStateException("Test working directory must be empty.");
+            }
+        }
+    }
+
+    @BeforeEach
+    public void cleanUpTestDirectory() throws IOException {
+        toolBox.cleanDirectory(Path.of("."));
+    }
+
+    @Test
+    public void jarFileNotModifiedOrdinaryCompilation() throws IOException {
+        byte[] originalJarBytes = compileTestLibJar();
+
+        new JavacTask(toolBox)
+                .sources(TARGET_SOURCE)
+                .classpath(TEST_LIB_JAR)
+                .run()
+                .writeAll();
+
+        // Assertion 1: The JAR is unchanged.
+        assertArrayEquals(originalJarBytes, Files.readAllBytes(TEST_LIB_JAR), "Jar file was modified.");
+        // Assertion 2: An output class file was written to the current directory.
+        assertTrue(Files.exists(OUTPUT_CLASS_FILE), "Output class file missing.");
+    }
+
+    // As above, but the JAR is added to the source path instead (with same results).
+    @Test
+    public void jarFileNotModifiedForSourcePath() throws IOException {
+        byte[] originalJarBytes = compileTestLibJar();
+
+        new JavacTask(toolBox)
+                .sources(TARGET_SOURCE)
+                .sourcepath(TEST_LIB_JAR)
+                .run()
+                .writeAll();
+
+        // Assertion 1: The JAR is unchanged.
+        assertArrayEquals(originalJarBytes, Files.readAllBytes(TEST_LIB_JAR), "Jar file was modified.");
+        // Assertion 2: An output class file was written to the current directory.
+        assertTrue(Files.exists(OUTPUT_CLASS_FILE), "Output class file missing.");
+    }
+
+    @Test
+    public void jarFileNotModifiedAnnotationProcessing() throws IOException {
+        byte[] originalJarBytes = compileTestLibJar();
+
+        new JavacTask(toolBox)
+                .sources(TARGET_SOURCE)
+                .classpath(TEST_LIB_JAR)
+                .processors(new TestAnnotationProcessor())
+                // Use "-implicit:none" to avoid writing the library class file.
+                .options("-implicit:none", "-g:source,lines,vars")
+                .run()
+                .writeAll();
+
+        // Assertion 1: The JAR is unchanged.
+        assertArrayEquals(originalJarBytes, Files.readAllBytes(TEST_LIB_JAR), "Jar file was modified.");
+        // Assertion 2: All expected output files were written to the current directory.
+        assertDummyFile("DummySource.java");
+        assertDummyFile("DummyClass.class");
+        assertDummyFile("DummyResource.txt");
+        // Assertion 3: The class file itself wasn't written (because we used "-implicit:none").
+        assertFalse(Files.exists(OUTPUT_CLASS_FILE), "Unexpected class file in working directory.");
+    }
+
+    static class TestAnnotationProcessor extends JavacTestingAbstractProcessor {
+        @Override
+        public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment env) {
+            // Only run this once (in the final pass), or else we get spurious failures about
+            // trying to recreate file objects (not allowed during annotation processing).
+            if (!env.processingOver()) {
+                return false;
+            }
+
+            TypeElement libClass = elements.getTypeElement(LIB_CLASS_TYPE_NAME);
+            try {
+                // Note: A generated Java source file must be legal Java, but a generated class
+                // file that's unreferenced will never be loaded, so can contain any bytes.
+                writeFileObject(
+                        filer.createSourceFile("DummySource", libClass),
+                        "DummySource.java",
+                        "class DummySource {}");
+                writeFileObject(
+                        filer.createClassFile("DummyClass", libClass),
+                        "DummyClass.class",
+                        "<<DummyClass Bytes>>");
+                writeFileObject(
+                        filer.createResource(CLASS_OUTPUT, "", "DummyResource.txt", libClass),
+                        "DummyResource.txt",
+                        "Dummy Resource Bytes");
+            } catch (IOException e) {
+                throw new RuntimeException(e);
+            }
+            return false;
+        }
+    }
+
+    static void writeFileObject(FileObject file, String expectedName, String contents)
+            throws IOException {
+        URI fileUri = file.toUri();
+        // Check that the file URI doesn't look like it is associated with a JAR.
+        assertTrue(fileUri.getSchemeSpecificPart().endsWith("/" + expectedName));
+        // The JAR file system would have a scheme of "jar", not "file".
+        assertEquals("file", fileUri.getScheme());
+        // Testing a negative is fragile, but a JAR URI would be expected to contain "jar!".
+        assertFalse(fileUri.getSchemeSpecificPart().contains("jar!"));
+        // Write dummy data (which should end up in the test working directory).
+        try (OutputStream os = file.openOutputStream()) {
+            os.write(contents.getBytes());
+        }
+    }
+
+    static void assertDummyFile(String filename) throws IOException {
+        Path path = Path.of(filename);
+        assertTrue(Files.exists(path), "Output file missing: " + filename);
+        assertTrue(Files.readString(path).contains("Dummy"), "Unexpected file contents: " + filename);
+    }
+
+    // Compiles and writes the test library JAR (LIB_JAR) into the current directory.
+    static byte[] compileTestLibJar() throws IOException {
+        Path libDir = Path.of("lib");
+        toolBox.createDirectories(libDir);
+        try {
+            toolBox.writeFile(LIB_SOURCE_FILE_NAME,
+                    """
+                    package lib;
+                    public class LibClass {
+                        public static final String OLD_FIELD = "This will not compile with Target";
+                    }
+                    """);
+
+            // Compile the old (broken) source and then store the class file in the JAR.
+            // The class file is generated in the lib/ directory, which we delete after
+            // making the JAR. This ensures that when compiling the target class, it's
+            // the source file being read from the JAR,
+            new JavacTask(toolBox)
+                    .files(LIB_SOURCE_FILE_NAME)
+                    .run()
+                    .writeAll();
+
+            // If timestamps are equal JAR file resolution of classes is ambiguous
+            // (currently "last one wins"), so give the source we want to be used a
+            // newer timestamp.
+            Instant now = Instant.now();
+            try (OutputStream jos = Files.newOutputStream(Path.of("lib.jar"))) {
+                JarOutputStream jar = new JarOutputStream(jos);
+                writeEntry(jar,
+                        LIB_SOURCE_FILE_NAME,
+                        """
+                        package lib;
+                        public class LibClass {
+                            public static final String NEW_FIELD = "This will compile with Target";
+                        }
+                        """.getBytes(StandardCharsets.UTF_8),
+                        now.plusSeconds(1));
+                writeEntry(jar,
+                        LIB_CLASS_FILE_NAME,
+                        Files.readAllBytes(Path.of(LIB_CLASS_FILE_NAME)),
+                        now);
+                jar.close();
+            }
+            // Return the JAR file bytes for comparison later.
+            return Files.readAllBytes(TEST_LIB_JAR);
+        } finally {
+            toolBox.cleanDirectory(libDir);
+            toolBox.deleteFiles(libDir);
+        }
+    }
+
+    // Note: JarOutputStream only writes modification time, not creation time, but
+    // that's what Javac uses to determine "newness" so it's fine.
+    private static void writeEntry(JarOutputStream jar, String name, byte[] bytes, Instant timestamp)
+            throws IOException {
+        ZipEntry e = new ZipEntry(name);
+        e.setLastModifiedTime(FileTime.from(timestamp));
+        jar.putNextEntry(e);
+        jar.write(bytes);
+        jar.closeEntry();
+    }
+}