Java Class getDeclaredAnnotation()用法及代碼示例

如果此類中存在聲明的注釋，則使用java.lang.Class類的getDeclaredAnnotation()方法獲取指定的聲明的注釋類型的聲明的注釋。該方法以對象的形式返回該類。

用法:

public T getDeclaredAnnotation(Class<T> declared annotationClass)

參數：此方法接受一個聲明的注釋類型的參數，該參數是要獲取的聲明注釋的類型。

返回值：此方法返回聲明的注釋類的指定對象。

異常：該方法拋出：

NullPointerException ：如果給定的聲明注釋類為null。

下麵的程序演示了getDeclaredAnnotation()方法。

範例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)); 
    } 
}

輸出：

Class represented by myClass:class Test
DeclaredAnnotation of myClass:@DeclaredAnnotation(key=GFG, value=GeeksForGeeks)

範例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); 
        } 
    } 
}

輸出：

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

參考： https://docs.oracle.com/javase/9/docs/api/java/lang/Class.html#getDeclaredAnnotation-java.lang.Class-

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