fix: custom shell with jsr/ret generate failed

generator/src/main/java/com/reajason/javaweb/memshell/generator/ByteBuddyShellGenerator.java

@@ -23,6 +23,10 @@ public abstract class ByteBuddyShellGenerator<T extends ShellToolConfig> impleme
 
     protected abstract DynamicType.Builder<?> getBuilder();
 
+    protected int getTargetJreVersion() {
+        return shellConfig.getTargetJreVersion();
+    }
+
     protected byte[] postProcessBytes(byte[] classBytes) {
         return classBytes;
     }
@@ -43,7 +47,7 @@ public abstract class ByteBuddyShellGenerator<T extends ShellToolConfig> impleme
 
         builder = ProcessorRegistry.applyBuilderProcessors(builder, shellConfig, shellToolConfig)
                 .name(shellClassName)
-                .visit(new TargetJreVersionVisitorWrapper(shellConfig.getTargetJreVersion()));
+                .visit(new TargetJreVersionVisitorWrapper(getTargetJreVersion()));
 
         try (DynamicType.Unloaded<?> unloaded = builder.make()) {
             byte[] bytes = postProcessBytes(unloaded.getBytes());

generator/src/main/java/com/reajason/javaweb/memshell/generator/CustomShellGenerator.java

@@ -7,6 +7,10 @@ import net.bytebuddy.description.type.TypeDescription;
 import net.bytebuddy.dynamic.ClassFileLocator;
 import net.bytebuddy.dynamic.DynamicType;
 import net.bytebuddy.jar.asm.ClassReader;
+import net.bytebuddy.jar.asm.ClassVisitor;
+import net.bytebuddy.jar.asm.Label;
+import net.bytebuddy.jar.asm.MethodVisitor;
+import net.bytebuddy.jar.asm.Opcodes;
 import net.bytebuddy.pool.TypePool;
 import org.apache.commons.lang3.StringUtils;
 
@@ -17,6 +21,7 @@ import java.util.Base64;
  * @since 2025/3/18
  */
 public class CustomShellGenerator extends ByteBuddyShellGenerator<CustomConfig> {
+    private boolean containsSubroutines;
 
     public CustomShellGenerator(ShellConfig shellConfig, CustomConfig customConfig) {
         super(shellConfig, customConfig);
@@ -27,6 +32,7 @@ public class CustomShellGenerator extends ByteBuddyShellGenerator<CustomConfig>
         String shellClassBase64 = shellToolConfig.getShellClassBase64();
         byte[] classBytes = Base64.getDecoder().decode(shellClassBase64);
         ClassReader classReader = new ClassReader(classBytes);
+        containsSubroutines = containsSubroutines(classReader);
         String className = classReader.getClassName().replace('/', '.');
         if (StringUtils.isBlank(shellToolConfig.getShellClassName())) {
             shellToolConfig.setShellClassName(className);
@@ -43,4 +49,38 @@ public class CustomShellGenerator extends ByteBuddyShellGenerator<CustomConfig>
         return new ByteBuddy()
                 .redefine(typeDescription, compoundLocator);
     }
+
+    @Override
+    protected int getTargetJreVersion() {
+        // Byte Buddy cannot emit a class version newer than Java 5 when the
+        // source bytecode contains legacy jsr/ret subroutines. Keep such
+        // custom classes at Java 5; Java 6+ runtimes can load them as well.
+        return containsSubroutines ? Opcodes.V1_5 : super.getTargetJreVersion();
+    }
+
+    private static boolean containsSubroutines(ClassReader classReader) {
+        final boolean[] found = {false};
+        classReader.accept(new ClassVisitor(Opcodes.ASM9) {
+            @Override
+            public MethodVisitor visitMethod(int access, String name, String descriptor,
+                                             String signature, String[] exceptions) {
+                return new MethodVisitor(Opcodes.ASM9) {
+                    @Override
+                    public void visitJumpInsn(int opcode, Label label) {
+                        if (opcode == Opcodes.JSR) {
+                            found[0] = true;
+                        }
+                    }
+
+                    @Override
+                    public void visitVarInsn(int opcode, int var) {
+                        if (opcode == Opcodes.RET) {
+                            found[0] = true;
+                        }
+                    }
+                };
+            }
+        }, ClassReader.SKIP_DEBUG | ClassReader.SKIP_FRAMES);
+        return found[0];
+    }
 }