Java 中的类 getDeclaredAnnotation()方法，带示例

原文:https://www . geesforgeks . org/class-getdeclaredannotation-method-in-Java-with-examples/

java.lang.Class 类 的 getDeclaredAnnotation() 方法用于获取指定声明注释类型的声明注释，如果该类中存在这样的声明注释。方法以对象的形式返回该类。

语法:

public T getDeclaredAnnotation(Class<T> declared annotationClass)

参数:该方法接受一个参数声明的注释类，它是要获取的声明注释的类型。

返回值:该方法返回声明的标注类的指定对象。

异常:此方法抛出:

  • NullPointRexception:如果给定的声明注释类为空。

下面的程序演示了 getDeclaredAnnotation()方法。

例 1:

// Java program to demonstrate
// getDeclaredAnnotation() method

import java.util.*;
import java.lang.annotation.*;

// create a custom DeclaredAnnotation
@Retention(RetentionPolicy.RUNTIME)
@interface DeclaredAnnotation {

    // This declared annotation has two attributes.
    public String key();

    public String value();
}

// call DeclaredAnnotation for method
// and pass values for declared annotation
@DeclaredAnnotation(key = "GFG", value = "GeeksForGeeks")
public class Test {

    public Object obj;

    public static void main(String[] args)
        throws ClassNotFoundException
    {

        // returns the Class object for this class
        Class myClass = Test.class;

        System.out.println(
            "Class represented by myClass: "
            + myClass.toString());

        // Get the declared annotation
        // using getDeclaredAnnotation() method
        System.out.println(
            "DeclaredAnnotation of myClass: "
            + myClass
                  .getDeclaredAnnotation(
                      DeclaredAnnotation.class));
    }
}

输出:

由 myClass 表示的类:class Test DeclaredAnnotation of my class:@ DeclaredAnnotation(键=GFG，值=GeeksForGeeks)

例 2:

// Java program to demonstrate
// getDeclaredAnnotation() method

import java.util.*;
import java.lang.annotation.*;

@Deprecated
public class Test {

    public Object obj;

    public static void main(String[] args)
        throws ClassNotFoundException
    {
        try {
            // returns the Class object for this class
            Class myClass = Test.class;

            System.out.println(
                "Class represented by myClass: "
                + myClass.toString());

            // Get the declared annotation
            // using getDeclaredAnnotation() method
            System.out.println(
                "DeclaredAnnotation of myClass: "
                + myClass
                      .getDeclaredAnnotation(
                          Deprecated.class));
        }
        catch (Exception e) {
            System.out.println(e);
        }
    }
}

输出:

Class represented by myClass: class Test
DeclaredAnnotation of myClass: @java.lang.Deprecated()

参考:https://docs . Oracle . com/javase/9/docs/API/Java/lang/class . html # getdeclarredannotation-Java . lang . class-

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