What is the principle of java reflection mechanism

Java reflection mechanism principle: The source code we write is a .java file. After being compiled by javac, it becomes a .class file, that is, a bytecode file. When the program is executed, the JVM will class load the bytecode file into the memory. , strictly speaking, it is loaded into the method area and converted into a java.lang.Class object. Through reflection, we can build an instance, get the value of the member variable, get the method and call it.

#The operating environment of this article: Windows 10 system, java17 version, dell g3 computer.

The principle of reflection is:

The source code we write is a .java file, which becomes a .class file after being compiled by javac, that is, a bytecode file.

When the program is executed, the JVM will load the bytecode file into the memory. Strictly speaking, it will be loaded into the method area and converted into a java.lang.Class object. Everything is an object. Class is called a class object, which describes the data structure of a class in the metadata space, including the attributes, methods, constructors, annotations, interfaces and other information defined in the class.

Through reflection, we can build instances, get the values ​​of member variables, get methods and call them. You can also get annotations defined on member variables, methods, and method parameters.

Implementation code:

1) Construct a parameterless instance: call the parameterless constructor through reflection

//1.通过全类名加载字节码对象
Class clazz = Class.forName("com.example.lib.Person");
//2.通过类的字节码拿到定义的构造函数
Constructor constructor = clazz.getConstructor();
//3.通过构造方法创建对象
Object obj = constructor.newInstance();

2) Construct a parameterized instance:

//1.通过全类名加载字节码对象
Class clazz = Class.forName("com.example.lib.Person");
//2.通过类的字节码拿到定义的构造函数
Constructor constructor = clazz.getConstructor(int.class,String.class);
//3.通过构造方法创建对象
Object obj = constructor.newInstance(20,"xiaohua");

3) Obtain the value of member variables through reflection.

//4.通过属性名获取属性
Field field = clazz.getDeclaredField("age");
field.setAccessible(true);
//5.调用get方法拿到对象obj属性age的值
Integer age = (Integer) field.get(obj);

4) Call the object's method through reflection.

//4.通过方法名和参数类型，拿到方法
method = clazz.getMethod("setAge", int.class);
//5.调用方法 obj是哪个对象身上的方法。
method.invoke(obj, 21);
method =  clazz.getMethod("getAge");
method.invoke(obj);

5) Obtain the value of static variables through reflection.

//1.通过全类名加载字节码对象
 Class clazz = Class.forName("com.example.lib.Person");
 //2.获取静态属性ID
 Field  field = clazz.getField("ID");
 field.setAccessible(true);
 //3.拿到静态属性ID的值。
 // 因为静态变量存在方法区，在对象创建之前，就已经加装到了内存
 //所以，没有对象，也可以获取变量的值，这里传null也是可以的。
 int id = (int) field.get(null);

Get the value of the defined annotation through reflection

1) Get the annotation and value of the member variable.

@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface BindView {
    int value();
}

public class MainActivity {
    @BindView(10000)
    TextView textView;
}

//10通过反射拿到定义在属性上的注解
Class  clazz = MainActivity.class;
Field textView = clazz.getDeclaredField("textView");
BindView bindView = textView.getAnnotation(BindView.class);
int txtId = bindView.value();

3) Obtain the annotations and values ​​defined on the method and method parameters through reflection

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Post {
    String value() default "";
}

public interface NetWorkInterface {
    @Post("http://www.baidu.com")
    Call getPerson(@Queue("name") String name, @Queue("200") int price);
}

//11通过反射拿到方法上定义的注解
  clazz = NetWorkInterface.class;
  Method method = clazz.getMethod("getPerson", String.class, int.class);
  //获取Post注解
  Post post = method.getAnnotation(Post.class);
  //获取值
  String url = post.value();

//12通过反射拿到参数上的注解
//为是个二维数组，因为方法参数会有多个，一个参数有可能定义多个注解
Annotation[][] annotations = method.getParameterAnnotations();
for (Annotation[] ans : annotations) {
    for (Annotation an : ans) {
        if (an instanceof Queue) {
            Queue queue = (Queue) an;
            String value = queue.value();
        }
    }
}

4) Obtain the parameter and return value types of the method.

//13.拿到方法参数的类型。
Type[] types = method.getGenericParameterTypes();
for (Type type : types) {
    System.out.println(type.toString());
}
//14.获取方法返回值类型
Type type = method.getGenericReturnType();

Summary:

Through reflection, you can get the values ​​of member variables on the object, call methods, and obtain member variables, methods and methods defined in the object. Annotations on parameters. Retrofit uses annotation plus reflection technology and dynamic proxy.

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