"Learn to the core principle of the JannoCessor framework: the meta -programming weapon in the Java class library"

Understand the core principle of the JannoCessor frame JannoCessor is a powerful Java code generation framework. It uses meta -injection and annotation processors to achieve flexible and efficient source code generation.This article will introduce the core principles of JannoCessor, explaining how it becomes an important tool for metad programming in the Java class library. Introduction to JannoCessor framework JannoCessor is a code generation framework built by the Java standard annotation API (javax.annotation.processing).It allows developers to generate source code during compilation to meet specific needs.By adding custom annotations to the Java class, JannoCessor can generate the source code corresponding to it by using the annotation and using the annotation processor. Second, the steps to use JannoCessor 1. Definition custom annotation Before using JannoCessor, we need to define custom annotations.These custom annotations will be used to mark the class or methods we want to generate code.For example, we can define a annotation called @Generatecode to indicate that JannoCessor generates code. 2. Create an annotation processor Next, we need to write an annotation processor to handle custom annotations.The annotation processor is the core component of the JannoCessor framework, which will generate the corresponding source code according to the definition of the annotation and the rules.We can create a custom annotation processor by inheriting the abstract class provided by the JannoCessor, and covering the logic of the source code generation. 3. Configuration compiler In order to enable the annotation processor, we need to configure the Java compiler during compilation.You can use the command line parameters or constructing tools (such as Maven or Gradle) provided by the Java compiler to set the annotation processor. 4. Compile and generate code After completing the above steps, we can use the Java compiler to compile code containing custom annotations.During the compilation process, the annotation processor will be automatically triggered to generate the corresponding source code file. Third, Example: Use JannoCessor to generate the builder mode code Below is an example of generating the BUILDER mode code using JannoCessor.We define an annotation called @Builder to mark the classes that need to generate the BUILDER mode code. public @interface Builder { } Next, we write an annotation processor to process @Builder annotations and generate the corresponding Builder mode code.Code examples are as follows: @SupportedAnnotationTypes({"com.example.Builder"}) @SupportedSourceVersion(SourceVersion.RELEASE_8) public class BuilderProcessor extends AbstractProcessor { @Override public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) { for (TypeElement annotation : annotations) { for (Element element : roundEnv.getElementsAnnotatedWith(annotation)) { // The logic of generating the BUILDER mode code } } return true; } } After completing the above steps, we can use @Builder annotations in our Java class to generate the corresponding Builder mode code.For example: @Builder public class MyClass { private String name; private int age; } After compiling a class that includes @Builder annotations, JannoCessor will automatically trigger the BuilderProcessor annotation processor to generate the following Builder mode code: public class MyClassBuilder { private String name; private int age; public MyClassBuilder setName(String name) { this.name = name; return this; } public MyClassBuilder setAge(int age) { this.age = age; return this; } public MyClass build() { return new MyClass(name, age); } } By using the JannoCessor framework, we can easily realize the need to generate code, improve development efficiency and reduce duplicate labor. Summarize The JannoCessor framework is a powerful Java code generation tool. Using the meta -solution and annotation processor it provided, we can generate various code during the compilation period, which greatly improves development efficiency.This article briefly introduces the core principles of JannoCessor, and demonstrates the use of JannoCessor to generate the Builder mode code as an example.It is hoped that readers can understand the JannoCessor framework through this article and use it flexibly in actual development.