Java Class getDeclaredConstructor()用法及代碼示例

java.lang.Class類的getDeclaredConstructor()方法用於獲取具有指定參數類型的此類的指定構造函數。該方法以Constructor對象的形式返回此類的指定構造函數。

用法:

public Constructor<T>
       getDeclaredConstructor(Class[] parameterType)
       throws NoSuchMethodException, SecurityException

參數：此構造函數接受參數parameterType，它是指定構造函數的參數類型的數組。

返回值：此方法以Constructor對象的形式返回此類的指定構造函數。

異常該方法拋出：

• NoSuchMethodException如果找不到具有指定名稱的構造函數。
• SecurityException如果存在安全管理員並且不滿足安全條件。

  下麵的程序演示了getDeclaredConstructor()方法。

  示例1:

  // Java program to demonstrate 
  // getDeclaredConstructor() method 
    
  import java.util.*; 
    
  public class Test { 
    
      public Test() {} 
    
      public static void main(String[] args) 
          throws ClassNotFoundException, NoSuchMethodException 
      { 
    
          // returns the Class object for this class 
          Class myClass = Class.forName("Test"); 
    
          System.out.println("Class represented by myClass: "
                             + myClass.toString()); 
    
          Class[] parameterType = null; 
    
          // Get the constructor of myClass 
          // using getDeclaredConstructor() method 
          System.out.println( 
              "Constructor of myClass: "
              + myClass.getDeclaredConstructor(parameterType)); 
      } 
  }

  輸出：

  Class represented by myClass: class Test
  Constructor of myClass: public Test()
  

  示例2:

  // Java program to demonstrate 
  // getDeclaredConstructor() constructor 
    
  import java.util.*; 
    
  class Main { 
    
      private Main() {} 
    
      public static void main(String[] args) 
          throws ClassNotFoundException, NoSuchMethodException 
      { 
    
          // returns the Class object for this class 
          Class myClass = Class.forName("Main"); 
    
          System.out.println("Class represented by myClass: "
                             + myClass.toString()); 
    
          Class[] parameterType = new Class[1]; 
          parameterType[0] = Long.class; 
    
          try { 
              // Get the constructor of myClass 
              // using getDeclaredConstructor() method 
              System.out.println( 
                  "Constructor of myClass: "
                  + myClass.getDeclaredConstructor(parameterType)); 
          } 
          catch (Exception e) { 
              System.out.println(e); 
          } 
      } 
  }

  輸出：

  Class represented by myClass: class Main
  java.lang.NoSuchMethodException: Main.(java.lang.Long)
  

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