8358078: javap crashes with NPE on preview class file

Reviewed-by: liach

src/jdk.jdeps/share/classes/com/sun/tools/javap/ClassWriter.java

@@ -118,9 +118,11 @@ public class ClassWriter extends BasicWriter {
     protected ClassFileFormatVersion cffv() {
         var major = classModel.majorVersion();
         if (major < JAVA_1_VERSION || major > ClassFile.latestMajorVersion())
-            return null;
+            // something not representable by CFFV, let's fall back
+            return ClassFileFormatVersion.latest();
         if (major >= JAVA_12_VERSION && classModel.minorVersion() != 0) {
-            return null;
+            // preview versions aren't explicitly supported, but latest is good enough for now
+            return ClassFileFormatVersion.latest();
         }
         return ClassFileFormatVersion.fromMajor(major);
     }

test/langtools/tools/javap/ClassFileVersionTest.java

@@ -0,0 +1,98 @@
+/*
+ * 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 8358078
+ * @summary javap should not crash due to class file versions
+ * @library /tools/lib
+ * @modules jdk.jdeps/com.sun.tools.javap
+ * @run junit ClassFileVersionTest
+ */
+
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+import toolbox.JavapTask;
+import toolbox.Task;
+import toolbox.ToolBox;
+
+import java.lang.classfile.ClassFile;
+import java.lang.constant.ClassDesc;
+import java.lang.reflect.AccessFlag;
+import java.lang.reflect.ClassFileFormatVersion;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.stream.Stream;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.params.provider.Arguments.of;
+
+public class ClassFileVersionTest {
+
+    final ToolBox toolBox = new ToolBox();
+
+    public static Stream<Arguments> classFiles() {
+        int major17 = ClassFileFormatVersion.RELEASE_17.major();
+        int preview = Character.MAX_VALUE;
+        int majorLatest = ClassFileFormatVersion.latest().major();
+        AccessFlag[] noFlags = {};
+        return Stream.of(
+            of(false, major17, 0, noFlags),
+            of(false, major17, preview, noFlags),
+            of(false, 0, 0, noFlags),
+            of(false, major17, 0, new AccessFlag[]{AccessFlag.PUBLIC}),
+            of(false, major17, preview, new AccessFlag[]{AccessFlag.PUBLIC}),
+            of(false, majorLatest, preview, new AccessFlag[]{AccessFlag.PUBLIC}),
+            of(true, majorLatest, 0, new AccessFlag[]{AccessFlag.BRIDGE}), // misplaced access flag
+            of(true, majorLatest, preview, new AccessFlag[]{AccessFlag.BRIDGE}) // misplaced access flag
+        );
+    }
+
+    private static byte[] createClassFile(int major, int minor, AccessFlag[] classFlags) {
+        return ClassFile.of().build(ClassDesc.of("Test"), (builder) -> {
+            // manually assemble flag bits to avoid exception in ClassFile api
+            int flags = 0;
+            for (AccessFlag classFlag : classFlags) {
+                flags |= classFlag.mask();
+            }
+            builder.withVersion(major, minor).withFlags(flags);
+        });
+    }
+
+    @ParameterizedTest
+    @MethodSource("classFiles")
+    void test(boolean shouldError, int major, int minor, AccessFlag[] classFlags) throws Throwable {
+
+        Files.write(Path.of("cf.class"), createClassFile(major, minor, classFlags));
+
+        var lines = new JavapTask(toolBox)
+            .classes("cf.class")
+            .options("-c", "-p", "-v")
+            .run(shouldError ? Task.Expect.FAIL : Task.Expect.SUCCESS)
+            .writeAll()
+            .getOutputLines(Task.OutputKind.DIRECT);
+
+        assertEquals(shouldError, lines.stream().anyMatch(l -> l.startsWith("Error: Access Flags:")), "printed error");
+    }
+}