JAVA基础知识之反射

以下是我总结的一些简单的JAVA反射相关的小例子，非常简单，一看您就明白了，有什么问题您评论，我解答。

基础bean

public class Person {
    public int age;
    public String name;
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Person(String name,int age) {
        this.age = age;
        this.name = name;
    }
    public Person() {
    }
}

三种获取Class的方式

//第一种
Class<?> helloClass = "hello".getClass();
Class<? extends String> helloClassNew = "hello".getClass();
Class<? extends Object> helloClassNew2 = "hello".getClass();
//第二种
Class<?> stringClass = String.class;
//第三种
String className ="java.lang.String";
Class<?> stringClassNew = Class.forName(className);

获取父类型以及实现的接口类型

Class<?> superClass = stringClass.getSuperclass();
Class<?>[] interfaces = stringClass.getInterfaces();

获取类的字段

Class<?> clss = Person.class;
Field field = clss.getField("age");
//获取该类也同时包括父类型的全部public字段
Field[] declaredFields = clss.getDeclaredFields();
//获取该类内部的全部字段，无论private还是public
Field[] fields = clss.getFields();

获取方法

Method method =clss.getMethod("setName", String.class);
//获取该类也同时包括父类型的全部public方法
Method[] declaredMethods = clss.getDeclaredMethods();
//获取所有该类内部的method，无论private还是public
Method[] methods = clss.getMethods();

获取构造函数

Constructor<?> constructor = clss.getConstructor(Class<?> ..types);
//获取该类内部的所有构造函数
Constructor<?>[] declaredConstructor = clss.getDeclaredConstructors();
//获取该类内部的所有public构造函数
Constructor<?>[] constructors = clss.getConstructors();

获取修饰

int modifiers = constructor.getModifiers();
boolean isPublic = Modifier.isPublic(modifiers);
设置字段为public的值，字段age为public
Person person = new Person();
Class<?> o = person.getClass();
Field field = o.getDeclaredField("age");
field.set(person,"d");
System.out.println(person.getAge());
//输出
d

设置字段为private的值，字段name为private，需要增加setAccessible为true，如果不设置为true则会报IllegalAccessException

Person person = new Person();
Class<?> o = person.getClass();
Field field = o.getDeclaredField("name");
field.setAccessible(true);
field.set(person,"d");
System.out.println(person.getName());

通过获取字段的注解，做操作。

Person person = new Person();
Class<?> o = person.getClass();
Field[] fields = o.getDeclaredFields();
for(Field field:fields){
    if(field.getAnnotatedType(PrimaryKey.class) !=null){
        //this is the primary key
    }
    if(field.getAnnotatedType(Column.class) !=null){
        //this is an elment to read / write
    }
}

通过MethodHandles获取类

MethodHandles.Lookup lookup = MethodHandles.lookup();
Class<?> personClass = MethodHandles.lookup().findClass(Person.class.getName()); 

创建四种method

//getter方法
MethodType getterType = MethodType.methodType(String.class);
MethodHandle getterHandle = lookup.findVirtual(Person.class,"getName",getterType);
//setter方法
MethodType setterType = MethodType.methodType(void.class,String.class);
MethodHandle setterHandle = lookup.findVirtual(Person.class,"setName",setterType);
//构造方法
MethodType constructorType = MethodType.methodType(void.class,String.class,int.class);
MethodHandle constructorHandle = lookup.findConstructor(Person.class, constructorType);
//空构造方法
MethodType emptyConstructorType = MethodType.methodType(void.class);
MethodHandle emptyConstructorHandle = lookup.findConstructor(Person.class,emptyConstructorType);

通过invoke创建bean

MethodType constructorType = MethodType.methodType(void.class,String.class,int.class);
MethodHandle constructorHandle = lookup.findConstructor(Person.class, constructorType);

Person p = (Person)constructorHandle.invoke("Xuesong",33);
System.out.println(p);
//输出
Person{age=33, name='Xuesong'}

MethodType emptyConstructorType = MethodType.methodType(void.class);
MethodHandle emptyConstructorHandle = lookup.findConstructor(Person.class,emptyConstructorType);

Person p2 = (Person)emptyConstructorHandle.invoke();
System.out.println(p2);
//输出
Person{age=0, name='null'}

通过invoke调用getter和setter

MethodHandle nameReader = lookup.findGetter(Person.class, "name", String.class);
MethodHandle nameWriter = lookup.findSetter(Person.class,"name",String.class);
Person person = new Person("Xuesong",33);
String name = (String) nameReader.invoke(person);
//或者
//String name = (String) getterHandle.invoke(person);
System.out.println(name);
//输出
Xuesong
nameWriter.invoke(person,"John");
//或者
//setterHandle.invoke(person,"John");
System.out.println(person.getName());
//输出
John

针对private的字段需要特殊处理

Field nameField = Person.class.getDeclaredField("name");
nameField.setAccessible(true);
MethodHandle privateNameReader = lookup.unreflectGetter(nameField);
String name = (String) privateNameReader.invoke(person);
System.out.println(name);
MethodHandle privateNameWriter = lookup.unreflectSetter(nameField);
privateNameWriter.invoke(person,"John");
System.out.println(person.getName());
//JDK9以后的方式
Lookup privateLookup = MethodHandles.privateLookupIn(Person.class, lookup);
MethodHandle privateNameWriter = privateLookup.findSetter(Person.class,"name",String.class);
privateNameWriter.invoke(person, "John");

JDK9以后增加了线程安全的一种VarHandle，也可以处理

VarHandle nameVarHandle = MethodHandles.privateLookupIn(Person.class,lookup)
        .findVarHandle(Person.class,"name",String.class);
String name = (String) nameVarHandle.get(person);
String name = (String) nameVarHandle.getVolatile(person);