使用 Javassist 运行时生成泛型子类

1package cc.unmi;
2
3public abstract class Repository<T> {
4    abstract T findOne();
5}

1package cc.unmi;
2
3import java.lang.annotation.Retention;
4import static java.lang.annotation.RetentionPolicy.RUNTIME;
5
6@Retention(RUNTIME)
7public @interface Scope {
8    String value();
9}

 1package cc.unmi;
 2
 3import javassist.ClassPool;
 4import javassist.CtClass;
 5import javassist.CtConstructor;
 6import javassist.CtMethod;
 7import javassist.CtNewConstructor;
 8import javassist.CtNewMethod;
 9import javassist.bytecode.AnnotationsAttribute;
10import javassist.bytecode.ConstPool;
11import javassist.bytecode.SignatureAttribute;
12import javassist.bytecode.annotation.Annotation;
13import javassist.bytecode.annotation.StringMemberValue;
14
15public class Main {
16
17    @SuppressWarnings("unchecked")
18    public static void main(String[] args) throws Exception {
19        ClassPool pool = ClassPool.getDefault();
20        CtClass subClass = pool.makeClass("cc.unmi.UserRepository"); //类的全限名称
21        subClass.setSuperclass(pool.get(Repository.class.getName())); //指定父类，也可以在 makeClass() 的第二个参数指定
22
23        //Javassist 对泛型的支持不甚友好，实现方法中还是会把类型擦除
24        subClass.setGenericSignature(new SignatureAttribute.TypeVariable("Repository<String>").encode());
25
26        //即使隐式行为默认的构造函数调用父类的构造函数也必须说明
27        CtClass[] params = new CtClass[]{ };
28        CtConstructor ctor = CtNewConstructor.make( params, null, CtNewConstructor.PASS_PARAMS, null, null, subClass );
29        subClass.addConstructor(ctor);
30
31        //使用了源代码的方式来实现一个方法，注意这里的类型是被擦除的, 把 Object 改成 String 反而有问题，见后面的解释
32        CtMethod findOneMethod = CtNewMethod.make("public Object findOne(){return \"Yanbin\";}", subClass);
33        subClass.addMethod(findOneMethod);
34
35        //加个注解确实复杂，如果想一步创建 Annotation 用 new Annotation("Scope(value=\"Request\")", constPool)
36        //产生成类文件反编译后看起来也对的，但可能用反射 API 就是看不到它
37        ConstPool constPool = subClass.getClassFile().getConstPool();
38        AnnotationsAttribute annotationsAttribute = new AnnotationsAttribute(constPool, AnnotationsAttribute.visibleTag);
39        Annotation scopeAnnotation = new Annotation(Scope.class.getName(), constPool);
40        scopeAnnotation.addMemberValue("value", new StringMemberValue("Request", constPool));
41        annotationsAttribute.addAnnotation(scopeAnnotation);
42        subClass.getClassFile().addAttribute(annotationsAttribute);
43
44        //由于是 Maven 项目，所以写入到这个目录中，最后的类文件是 target/classes/cc/unmi/UserRepository.class
45        subClass.writeFile("target/classes");
46
47        Class<Repository<String>> repositoryClass = (Class<Repository<String>>) Class.forName("cc.unmi.UserRepository");
48        System.out.println(repositoryClass.getAnnotation(Scope.class).value()); //输出 Request
49
50        Repository<String> repository = repositoryClass.newInstance();
51        System.out.println(repository.findOne()); //输出 Yanbin
52
53    }
54}

 1//
 2// Source code recreated from a .class file by IntelliJ IDEA
 3// (powered by Fernflower decompiler)
 4//
 5
 6package cc.unmi;
 7
 8import cc.unmi.Scope;
 9
10@Scope("Request")
11public class UserRepository extends Repository<String> {
12    public UserRepository() {
13    }
14
15    public Object findOne() {
16        return "Yanbin";
17    }
18}