访问者模式

• Visitor
• 内部结构不变，访问方式扩展

  • interface Visitor {
        void visitCpu(CPU cpu);
    } 
    class Visitor1 implements Visitor {
        double price = 0.0;
        @Override
        void visitCpu(CPU cpu) {
            price += cpu.getPrice() * 0.9;
        }
    }
    class Computer {
        Part cpu;
        void accept(Visitor v) {
            this.cpu.accept(v);
        }
    }
    abstract class Part {
        abstract void accept(Visitor v);
        abstract double getPrice();
    }
    class CPU extends Part {
        @Override
        void accept(Visitor v){
            v.visitCpu(this);
        }
    }
     
    Visitor p = new Visitor1();
    new Computer().accept(p);
    p.price;

• Java类AST编译器Visitor, ASM

  • // 打印
    class ClassPrinter extends ClassVisitor {
        @Override
        MethodVisitor visitMethod(){
            print(name + "()");
            retrun null;
        }
    }
    ClassPrinter cp = new ClassPrinter();
    ClassReader cr = new ClassReader("java.lang.Runnable");
    cr.accept(cp, 0);
     
    // 生成类
    ClassWriter cw = new ClassWriter(0);
    cw.visitMethod(ACC_PULIC + ACC_ABSTRACT, "compareTo", "(Ljava/lang/Object;)I", null, null).visitEnd();
    cw.visitEnd();
    MyClassLoader cl = new MyClassLoader();
    byte[] b = cw.toByteArray();
    Class c = cl.defineClass("pkg.Comparable", b, 0, b.length);
     
    // 代理类
    ClassReader cr = new ClassReader();
    ClassWriter cw = new ClassWriter(0);
    ClassVisitor cv = new ClassVisitor(ASM4, cw) {
        @Override
        public MethodVisitor visitMethod() {
            MethodVisitor mv = super.visitMethod(); 
            return new MethodVisitor(ASM4, mv) {
                @Override
                public void visitCode(){
                    visitMethodInsn(INVOKESTATIC, "TimeProxy", "before", "()v", false);
                    super.visitCode();
                }
            }
        }
    };
    cr.accept(cv, 0);
    cw.toByteArray();