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Java Build in Annotation¶

Overview¶

Information for the compiler: Annotations can be used by the compiler to detect errors or suppress warnings.
Compile-time and deployment-time processing: Software tools can process annotation information to generate code, XML files, and so forth.
Runtime processing: Some annotations are available to be examined at runtime.

API¶

meta annotation

@Target
@Retention
@Documented
@Inherited

Target

PACKAGE: package
TYPE: class, interface, enumeration
CONSTRUCTOR:
METHOD:
FIELD:
LOCAL_VARABLE:

Retention

SOURCE:
CLASS:
RUNTIME: for reflection

Target + Retention¶

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Target(value = { ElementType.METHOD, ElementType.TYPE })
@Retention(RetentionPolicy.RUNTIME)
public @interface AnnotationDemo {
    String studentName() default ""; // 参数名，参数类型
    int age() default 0;
    int id() default -1; //String indexOf("abc")  -1

    String[] schools() default {"school1", "school2"};
}

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Target(value = { ElementType.METHOD, ElementType.TYPE })
@Retention(RetentionPolicy.RUNTIME)
public @interface AnnotationDemo02 {
    String value();
}

import java.util.ArrayList;
import java.util.Date;
import java.util.List;

@AnnotationDemo(age = 19, studentName = "sname", id = 10001, schools = { "school1, school2" })
public class Demo {
    @Override
    @AnnotationDemo
    public String toString() {
        return "";
    }

    @AnnotationDemo02(value = "aaaa")
    public void test2() {
    }

    @AnnotationDemo02("aaaa")
    public void test3() {
    }

    @Deprecated
    public static void test001() {
        System.out.println("test001");
    }

    @SuppressWarnings("all")
    public static void test002() {
        List list = new ArrayList();
        List list2 = new ArrayList();

    }

    @SuppressWarnings("all")
    public static void main(String[] args) {
        Date d = new Date();
        d.getDate();
        test001();
    }
}

Reflection Annotation¶

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Target(value = {ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
public @interface Table {
    String value();
}

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Target(value = { ElementType.FIELD })
@Retention(RetentionPolicy.RUNTIME)
public @interface Field {
    String columnName();
    String type();
    int length();
}

package annotation;

@Table("tb_student")
public class Student {
    @Field(columnName = "id", type = "int", length = 10)
    private int id;
    @Field(columnName = "studentName", type = "String", length = 10)
    private String studentName;
    @Field(columnName = "age", type = "int", length = 3)
    private int age;

    public SxtStudent() {

    }

    public SxtStudent(int id, String studentName, int age) {
        this.id = id;
        this.studentName = studentName;
        this.age = age;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getStudentName() {
        return studentName;
    }

    public void setStudentName(String studentName) {
        this.studentName = studentName;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

}

import java.lang.annotation.Annotation;
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

public class Demo {

    /**
     * @param args
     * @throws SecurityException
     * @throws NoSuchFieldException
     * @throws NoSuchMethodException
     * @throws IllegalAccessException
     * @throws InstantiationException
     * @throws InvocationTargetException
     * @throws IllegalArgumentException
     */
    @SuppressWarnings("all")
    public static void main(String[] args) throws NoSuchFieldException, SecurityException, NoSuchMethodException,
            InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException {
        try {
            String path = "annotation/Student";
            Class clazz = Class.forName("annotation.Student");

            // 获得名字
            System.out.println(clazz.getName()); // 获得包名 + 类名
            System.out.println(clazz.getSimpleName()); // 获得包名 + 类名

            // 获得属性信息
            Field[] fields = clazz.getFields();
            System.out.println(fields.length);

            Field[] private_and_all_fields = clazz.getDeclaredFields();
            System.out.println(private_and_all_fields.length);
            for (Field f : private_and_all_fields) {
                System.out.print(f.getName() + " ");
            }
            System.out.print("\n");
            Field f = clazz.getDeclaredField("studentName");
            System.out.println(f.getName());

            // 获得方法
            Method[] methods = clazz.getMethods();
            for (Method m : methods) {
                System.out.print(m.getName() + " ");
            }
            System.out.print("\n");
            Method m = clazz.getDeclaredMethod("getId", null);
            System.out.println(m.getName());
            m = clazz.getDeclaredMethod("setId", int.class);// 如果方法有参数，必须传参数类型
            System.out.println(m.getName());

            // 获得构造器信息
            Constructor[] cons = clazz.getDeclaredConstructors();
            for (Constructor c : cons) {
                System.out.println("构造器: " + c.getName() + " ");
            }
            Constructor c = clazz.getDeclaredConstructor(null);
            System.out.println("构造器: " + c.getName() + " ");
            c = clazz.getDeclaredConstructor(int.class, String.class, int.class);
            System.out.println("构造器: " + c.getName() + " ");

            clazz = Class.forName("annotation.Student");
            Student stu = (Student) clazz.newInstance();// 其实调用的无参的构造器
            System.out.println(stu);
            Constructor<Student> cstu = clazz.getConstructor(int.class, String.class, int.class);
            stu = cstu.newInstance(100, "呵呵", 28);
            System.out.println(stu);

            // 反射方法
            Method method = clazz.getDeclaredMethod("setId", int.class);
            method.invoke(stu, 98);
            System.out.println(stu.getId());

            // 反射API操作属性
            Field field = clazz.getDeclaredField("studentName");
            field.setAccessible(true);
            field.set(stu, "瓦哈啊哈");
            System.out.println(stu.getStudentName());

            Annotation[] annotations = clazz.getAnnotations();
            for (Annotation a : annotations) {
                System.out.println(a);
            }

            Table st = (Table) clazz.getAnnotation(Table.class);
            System.out.println(st.value());

            Field f0 = clazz.getDeclaredField("studentName");
            Field sfield = f0.getAnnotation(Field.class);
            System.out.println(sfield.columnName() + "--" + sfield.type() + "--" + sfield.length());

            // Jvm 创建一个Clas对象
            System.out.println(clazz);
            System.out.println(clazz.hashCode());

            Class clazz2 = Class.forName("annotation.SxtStudent");
            System.out.println(clazz2);
            System.out.println(clazz2.hashCode());
            // Jvm 创建一个Clas对象
            Class strClass = String.class;
            Class strClass2 = path.getClass();
            System.out.println(strClass.hashCode());
            System.out.println(strClass2.hashCode());

        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }

    }

}

Authors¶

Weiduo Sun -

License¶

This project is licensed under the MIT License - see the LICENSE.md file for details