Ensure downgrading classes also provides necessary stubs to consumers

Author: Col-E

recaf-core/src/main/java/software/coley/recaf/util/JavaDowngraderUtil.java

@@ -2,20 +2,29 @@
 
 import jakarta.annotation.Nonnull;
 import jakarta.annotation.Nullable;
+import org.objectweb.asm.ClassReader;
+import org.objectweb.asm.ClassVisitor;
 import org.objectweb.asm.ClassWriter;
+import org.objectweb.asm.MethodVisitor;
 import org.objectweb.asm.Type;
 import org.objectweb.asm.tree.ClassNode;
 import org.slf4j.Logger;
+import software.coley.recaf.RecafConstants;
 import software.coley.recaf.analytics.logging.Logging;
+import software.coley.recaf.info.JvmClassInfo;
 import software.coley.recaf.services.inheritance.InheritanceGraph;
 import software.coley.recaf.util.visitors.WorkspaceClassWriter;
+import software.coley.recaf.workspace.model.bundle.JvmClassBundle;
+import software.coley.recaf.workspace.model.resource.RuntimeWorkspaceResource;
 import xyz.wagyourtail.jvmdg.ClassDowngrader;
 import xyz.wagyourtail.jvmdg.cli.Flags;
 import xyz.wagyourtail.jvmdg.util.Pair;
+import xyz.wagyourtail.jvmdg.version.map.FullyQualifiedMemberNameAndDesc;
 import xyz.wagyourtail.jvmdg.version.map.MemberNameAndDesc;
 
 import java.io.IOException;
 import java.util.Arrays;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
@@ -29,6 +38,11 @@
  */
 public class JavaDowngraderUtil {
 	private static final Logger logger = Logging.get(JavaDowngraderUtil.class);
+	private static final String[] undesirableStubs = {
+			// Array of stubs (owner;name;desc) that we do not want to pass back to callers.
+			"Lxyz/wagyourtail/jvmdg/j18/stub/java_base/J_L_System;getProperty;(Ljava/lang/String;)Ljava/lang/String;",
+			"Lxyz/wagyourtail/jvmdg/j18/stub/java_base/J_L_System;getProperties;()Ljava/util/Properties;"
+	};
 
 	/**
 	 * Downgrade the provided classes.
@@ -74,6 +88,8 @@ public static void downgrade(int targetJavaVersion,
 
 		Flags flags = new Flags();
 		flags.classVersion = targetClassVersion;
+		for (String undesirableStub : undesirableStubs)
+			flags.debugSkipStub.add(FullyQualifiedMemberNameAndDesc.of(undesirableStub));
 
 		try (ClassDowngrader downgrader = new RecafClassDowngrader(flags, inheritanceGraph)) {
 			int maxClassFileVersion = downgrader.maxVersion();
@@ -94,6 +110,7 @@ public static void downgrade(int targetJavaVersion,
 					try {
 						Map<String, byte[]> downgraded = downgrader.downgrade(new AtomicReference<>(className), classBytes, false,
 								key -> getBytes(maxClassFileVersion, classes, key));
+						fillStubClasses(downgraded);
 						if (downgraded != null)
 							downgraded.forEach(transformConsumer);
 					} catch (Throwable t) {
@@ -118,6 +135,40 @@ private static byte[] getBytes(int maxClassFileVersion, @Nonnull Map<String, byt
 		return classBytes;
 	}
 
+	private static void fillStubClasses(@Nullable Map<String, byte[]> downgraded) {
+		if (downgraded == null || downgraded.isEmpty())
+			return;
+
+		// Find all referenced stubs from the downgraded classes.
+		Set<String> referencedStubs = new HashSet<>();
+		for (byte[] classBytes : downgraded.values()) {
+			ClassReader reader = new ClassReader(classBytes);
+			reader.accept(new ClassVisitor(RecafConstants.getAsmVersion()) {
+				public MethodVisitor visitMethod(int access, String name, String descriptor, String signature, String[] exceptions) {
+					return new MethodVisitor(RecafConstants.getAsmVersion()) {
+						public void visitFieldInsn(int opcode, String owner, String name, String descriptor) {
+							if (owner.startsWith("xyz/wagyourtail/jvmdg/") && owner.contains("/stub/"))
+								referencedStubs.add(owner);
+						}
+
+						public void visitMethodInsn(int opcode, String owner, String name, String descriptor, boolean isInterface) {
+							if (owner.startsWith("xyz/wagyourtail/jvmdg/") && owner.contains("/stub/"))
+								referencedStubs.add(owner.replace('/', '.'));
+						}
+					};
+				}
+			}, ClassReader.SKIP_DEBUG | ClassReader.SKIP_FRAMES);
+		}
+
+		// Add all referenced stubs to the downgraded output.
+		JvmClassBundle runtimeBundle = RuntimeWorkspaceResource.getInstance().getJvmClassBundle();
+		for (String referencedStub : referencedStubs) {
+			JvmClassInfo cls = runtimeBundle.get(referencedStub);
+			if (cls != null)
+				downgraded.put(cls.getName(), cls.getBytecode());
+		}
+	}
+
 	private static class RecafClassDowngrader extends ClassDowngrader {
 		private final InheritanceGraph inheritanceGraph;
 

recaf-core/src/test/java/software/coley/recaf/services/compile/JavacCompilerTest.java

@@ -72,7 +72,8 @@ public class HelloWorld {
 							public static void main(String[] args) {
 								if (args.length < 1)
 									return;
-								String amount = args[0];
+								String amountProperty = System.getProperty(args[0]);
+								String amount = amountProperty;
 						
 								// List.of should be replaced since it was added in Java 9
 								List<String> values = new ArrayList<>(switch (amount) {
@@ -123,6 +124,10 @@ else if (name.equals("removeLast"))
 				};
 			}
 		}, ClassReader.SKIP_FRAMES | ClassReader.SKIP_DEBUG);
+
+		// Assert the necessary stubs were added
+		assertNotNull(result.getCompilations().get("xyz/wagyourtail/jvmdg/j9/stub/java_base/J_U_List"), "Missing stub for Java 9: List.of");
+		assertNotNull(result.getCompilations().get("xyz/wagyourtail/jvmdg/j21/stub/java_base/J_U_List"), "Missing stub for Java 21: List.removeLast");
 	}
 
 	@Test

recaf-ui/src/main/java/software/coley/recaf/ui/control/popup/ChangeClassVersionPopup.java

@@ -1,5 +1,6 @@
 package software.coley.recaf.ui.control.popup;
 
+import atlantafx.base.theme.Styles;
 import com.google.common.util.concurrent.AtomicDouble;
 import jakarta.annotation.Nonnull;
 import jakarta.annotation.Nullable;
@@ -243,8 +244,15 @@ private GridPane createPane(@Nonnull String suffix, @Nonnull UpdateHandler versi
 
 		layout.add(labelVersion, 0, 0);
 		layout.add(versionCombo, 1, 0);
-		layout.add(applyButton, 0, 2, 2, 1);
-		layout.add(progressBar, 0, 3, 2, 1);
+
+		if (suffix.equals("down")) {
+			Label labelNotice = new BoundLabel(getBinding("java.targetversion.notice." + suffix));
+			labelNotice.getStyleClass().addAll(Styles.TEXT_SUBTLE, Styles.TEXT_ITALIC);
+			layout.add(labelNotice, 0, layout.getRowCount(), 2, 1);
+		}
+
+		layout.add(applyButton, 0, layout.getRowCount(), 2, 1);
+		layout.add(progressBar, 0, layout.getRowCount(), 2, 1);
 		layout.setPadding(new Insets(10));
 		layout.setAlignment(Pos.TOP_CENTER);
 		return layout;

recaf-ui/src/main/resources/translations/en_US.lang

@@ -386,6 +386,7 @@ java.savewitherrors=It seems like this is your first time making changes that re
 java.savewitherrors.title=Regarding recompile errors
 java.decompiler=Decompiler
 java.targetversion=Compile target version
+java.targetversion.notice.down=Downgrading will add stub classes to emulate missing APIs
 java.targetversion.auto=Match class file version
 java.targetdownsampleversion=Downsample target version
 java.targetdownsampleversion.disabled=Disabled