Java Reflection - Java Constructor Reflection

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The following four methods from the Class class get information about the constructors:

Constructor[] getConstructors()
Constructor[]  getDeclaredConstructors()
Constructor<T> getConstructor(Class...  parameterTypes)
Constructor<T> getDeclaredConstructor(Class...  parameterTypes)

The getConstructors() method returns all public constructors from current and super class.

The getDeclaredConstructors() method returns all declared constructors for current class.

The getConstructor(Class... parameterTypes) and getDeclaredConstructor(Class... parameterTypes) get the Constructor object by the parameter types.

Example

The following code shows how to do reflection on constructors.

import java.lang.reflect.Constructor;
import java.lang.reflect.Executable;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.lang.reflect.Parameter;
import java.util.ArrayList;
/*from  w  ww . ja  v a 2s .  c  o m*/
class MyClass<T> {

  public MyClass(int i, int j, String s) {

  }

  public MyClass(T t) {

  }

  public int getInt(String a) {
    return 0;
  }
}

public class Main {
  public static void main(String[] args) {
    Class<MyClass> c = MyClass.class;

    System.out.println("Constructors for " + c.getName());
    Constructor[] constructors = c.getConstructors();
    ArrayList<String> constructDescList = getConstructorsDesciption(constructors);
    for (String desc : constructDescList) {
      System.out.println(desc);
    }
  }

  public static ArrayList<String> getConstructorsDesciption(
      Constructor[] constructors) {
    ArrayList<String> constructorList = new ArrayList<>();
    for (Constructor constructor : constructors) {
      String modifiers = getModifiers(constructor);

      String constructorName = constructor.getName();

      constructorList.add(modifiers + "  " + constructorName + "("
          + getParameters(constructor) + ") " + getExceptionList(constructor));
    }
    return constructorList;
  }
  public static ArrayList<String> getParameters(Executable exec) {
    Parameter[] parms = exec.getParameters();
    ArrayList<String> parmList = new ArrayList<>();
    for (int i = 0; i < parms.length; i++) {

      int mod = parms[i].getModifiers() & Modifier.parameterModifiers();
      String modifiers = Modifier.toString(mod);
      String parmType = parms[i].getType().getSimpleName();
      String parmName = parms[i].getName();
      String temp = modifiers + "  " + parmType + "  " + parmName;
      if (temp.trim().length() == 0) {
        continue;
      }
      parmList.add(temp.trim());
    }
    return parmList;
  }

  public static ArrayList<String> getExceptionList(Executable exec) {
    ArrayList<String> exceptionList = new ArrayList<>();
    for (Class<?> c : exec.getExceptionTypes()) {
      exceptionList.add(c.getSimpleName());
    }
    return exceptionList;
  }

  public static String getModifiers(Executable exec) {
    int mod = exec.getModifiers();
    if (exec instanceof Method) {
      mod = mod & Modifier.methodModifiers();
    } else if (exec instanceof Constructor) {
      mod = mod & Modifier.constructorModifiers();
    }
    return Modifier.toString(mod);
  }
}

The code above generates the following result.

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