Use the Spring ASM framework to perform the bytecode of the Java library to operate

Use the Spring ASM framework to perform the bytecode of the Java library to operate Summary: In Java development, sometimes we need to perform some bytecode operations on the existing classes, such as dynamic generation, modified behavior or annotations.The Spring framework provides a flexible and powerful bytecode operation tool that provides ASM (full name "Analysis of Statics As Method", which can be dynamically modified and generated by bytecode during runtime.This article will introduce how to use the Spring ASM framework for the byte code operation of the Java library. 1. What is the ASM framework? ASM is a framework based on the Java bytecode. It provides a convenient and efficient way to operate the byte code.It can be used to create, modify, and manipulate already existing bytecodes, enabling developers to generate and modify classes during runtime.Compared with other bytecode operating libraries, ASM has higher performance and simpler interface. 2. How to use the ASM framework for bytecode operation? The general steps of using the ASM framework for bytecode operation are as follows: 1. Introduce the Spring ASM library to the project.You can download the latest ASM library on Spring's official website and add it to your project. 2. Create ClassReader and ClassWriter objects.ClassReader is used to read existing types of bytecodes, and ClassWriter is used to generate new types of byte code. ClassReader Cr = New ClassReader (ClassName); // Read the existing type of bytecode ClassWriter CW = New ClassWriter (ClassWriter.comPute_frames); // Create a new type of byte code 3. Create the ClassVisitor object.ClassVisitor is one of the core interfaces of ASM. It is used to access and modify different parts of the class, such as class names, methods, fields, etc.You can inherit ClassVisitor to achieve customized access logic. ClassVisitor CV = New MyClassVisitor (CW); // Create a MyClassVisitor object, different parts of the processing class 4. Call the access () method of classReader and pass it into the ClassVisitor object.This will trigger ASM's various visit methods in classvisitor, so that you can access and modify different parts of the class. cr.accept(cv, ClassReader.SKIP_FRAMES); 5. Get the byte code generated.In the classVisitor's Visitend () method, you can obtain the generated new byte code by calling the CW.Tobytearray () method. byte[] newClassBytes = cw.toByteArray(); 6. Optional: Save the generated bytecode to the local file or load directly to the memory.You can save the generated bytecode to the local file, and use Java's ClassLoader to dynamically load the class, or use Java's Instrumentation API to load the class during runtime. // Save the byte code to the local file FileOutputStream fos = new FileOutputStream("newClass.class"); fos.write(newClassBytes); fos.close(); // Use ClassLoader dynamic loading class ClassLoader classLoader = new DynamicClassLoader(); Class<?> newClass = classLoader.defineClass("com.example.NewClass", newClassBytes, 0, newClassBytes.length); // Use Java Instrumentation API to load class premain() { ClassFileTransformer transformer = new MyClassFileTransformer(); Instrumentation instrumentation = ...; // Get the Instrumentation object instrumentation.addTransformer(transformer); instrumentation.retransformClasses(clazz); } 3. Advantages and application scenarios of ASM framework The ASM framework has the following advantages: -The high performance: ASM is a lightweight bytecode library. Compared with other bytecode operation tools, it has higher performance and lower memory occupation. -Feilicity: ASM provides a rich API, allowing developers to perform fine -granularity operations on the byte code, and can achieve many complex bytecode conversion and generating logic. -It is easy to use: ASM's API design is simple and clear, providing developers with intuitive and easy -to -understand ways to operate byte code. The ASM framework is suitable for the following application scenarios: -AOP (facing cut -oriented programming): By cutting in the byte code level, the cutting logic similar to Spring Aop can be achieved, such as method interceptors, log records, etc. -D dynamic proxy: Through dynamic generation or modified bytecode, dynamic proxy can be achieved, such as the PROXY mechanism and bytecode enhancement framework of Java, such as CGLIB. -Wotocode conversion: You can use the ASM framework to convert the existing bytecode, such as enhancement and deleting existing methods. in conclusion: The Spring ASM framework is a flexible and powerful bytecode operation tool that is suitable for dynamic production and modification of the library in Java development.Through ASM, developers can operate the bytecode at runtime to achieve some complex functions.I hope this article will help you understand and use the Spring ASM framework! references: - ASM: A Small and Safe Java Bytecode Manipulation Framework. http://asm.ow2.org/ - ASM User Guide. https://courses.cs.washington.edu/courses/cse401/01sp/sections/SectionSummary.pdf