Dynamic generation and runtime of bytecode

Dynamic generation and runtime control of bytecode Introduction: Bytecode is an intermediate representation form that translates the program into a instruction set that can be understood and executed by the Java virtual machine (JVM).In Java, the byte code is generated by compiling the Java source code, and it is cross -platform, because the Java virtual machines on different operating systems can read and execute the same byte code.This article will introduce how to generate the byte code dynamically during runtime, and to control the byte code to achieve flexible behavior. Dynamically generate byte code: In Java, some libraries or tools can be used to dynamically generate the byte code, including ASM, byte Buddy, and Javassist.Use these tools to create classes and methods as needed at runtime, and dynamically control these generated bytecodes.The following is an example of using ASM library to dynamically generate a simple class: import org.objectweb.asm.ClassWriter; import org.objectweb.asm.MethodVisitor; import org.objectweb.asm.Opcodes; public class DynamicClassGenerationExample { public static void main(String[] args) { // Create a ClassWriter instance for generating byte code ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_FRAMES | ClassWriter.COMPUTE_MAXS); // Define the basic information of the class cw.visit(Opcodes.V1_8, Opcodes.ACC_PUBLIC, "DynamicClass", null, "java/lang/Object", null); // Define the non -ginseng constructive method MethodVisitor constructor = cw.visitMethod(Opcodes.ACC_PUBLIC, "<init>", "()V", null, null); constructor.visitCode(); constructor.visitVarInsn(Opcodes.ALOAD, 0); constructor.visitMethodInsn(Opcodes.INVOKESPECIAL, "java/lang/Object", "<init>", "()V", false); constructor.visitInsn(Opcodes.RETURN); constructor.visitMaxs(1, 1); constructor.visitEnd(); // Definition custom method MethodVisitor method = cw.visitMethod(Opcodes.ACC_PUBLIC + Opcodes.ACC_STATIC, "sayHello", "()V", null, null); method.visitCode(); method.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;"); method.visitLdcInsn("Hello, Dynamic Generation!"); method.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false); method.visitInsn(Opcodes.RETURN); method.visitMaxs(2, 0); method.visitEnd(); // Get the generated byte code and load it to the memory byte[] generatedBytecode = cw.toByteArray(); MyClassLoader loader = new MyClassLoader(); Class<?> dynamicClass = loader.defineClass("DynamicClass", generatedBytecode); // Call the dynamic generation method try { dynamicClass.getMethod("sayHello").invoke(null); } catch (Exception e) { e.printStackTrace(); } } } // Custom ClassLoader is used to load dynamic generated classes class MyClassLoader extends ClassLoader { public Class<?> defineClass(String name, byte[] bytecode) { return defineClass(name, bytecode, 0, bytecode.length); } } Control the byte code when runtime: After the byte code dynamically generates the byte code, flexible behaviors can be performed by controlling the generated class and methods.The following is an example. Demonstration of how to modify the existing methods at runtime to change its behavior: import java.lang.instrument.ClassFileTransformer; import java.lang.instrument.IllegalClassFormatException; import java.lang.instrument.Instrumentation; import java.security.ProtectionDomain; public class RuntimeBytecodeManipulationExample { public static void main(String[] args) { // Register ClassFiletransFormer and specify conversion logic Instrumentation instrumentation = BytecodeTransformer.initialize(); // Define the existing method behavior behavior try { instrumentation.redefineClasses(new ClassDefinition(Sample.class, transform(Sample.class.getName()))); } catch (Exception e) { e.printStackTrace(); } // Create an instance and call the method to observe its output Sample sample = new Sample(); sample.sayhello (); // Output modified string } // Definition method conversion logic private static byte[] transform(String className) { if (className.equals(Sample.class.getName())) { return getModifiedClassfile(); } return null; } // The byte code of the modification method private static byte[] getModifiedClassfile() { // Use the byte code editing tool to generate the modified method byte code // Here try { ClassPool pool = ClassPool.getDefault(); CtClass ctClass = pool.get(Sample.class.getName()); CtMethod ctMethod = ctClass.getDeclaredMethod("sayHello"); ctMethod.setBody("{ System.out.println(\"Hello, Bytecode Manipulation!\"); }"); return ctClass.toBytecode(); } catch (Exception e) { e.printStackTrace(); } return null; } // Example class static class Sample { public void sayHello() { System.out.println("Hello, World!"); } } // Define the classfiletransformer class to modify the byte code during runtime static class BytecodeTransformer implements ClassFileTransformer { static Instrumentation initialize() { BytecodeTransformer transformer = new BytecodeTransformer(); Instrumentation instrumentation = BytecodeUtil.getInstrumentation(); instrumentation.addTransformer(transformer); return instrumentation; } @Override public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException { byte[] modifiedClassfile = transform(className); return modifiedClassfile != null ? modifiedClassfile : classfileBuffer; } } // Tool class, used to obtain an instance static class BytecodeUtil { static Instrumentation getInstrumentation() { return BytecodeAgent.instrumentation; } } // The independent agent class is used to obtain the Instrumentation instance static class BytecodeAgent { static Instrumentation instrumentation; public static void premain(String agentArgs, Instrumentation inst) { instrumentation = inst; } } } Through the above code, you can dynamically generate the byte code and control its behavior at runtime.The running results will output "Hello, Dynamic Generation!" And modified string "Hello, bytecode Manipulation!". The above is a brief introduction to the dynamic generation and operation of the byte code. I hope it will be helpful to you.