反射の基礎知識

Why do we need reflection?

Objectの実行期間のclassをチェックする
実行期間のコンストラクション関数
Object実行期間のメンバー変数とメソッドのチェック
Objectの実行期間を呼び出す方法

Example 1: Get class name from object

import java.lang.reflect.Method;

public class ReflectionHelloWorld {
	public static void main(String[] args){
		Foo f = new Foo();
		System.out.println(f.getClass().getName());			
	}
}

class Foo {
	public void print() {
		System.out.println("abc");
	}
}

 

Example 2: Invoke method on unknown object

import java.lang.reflect.Method;

public class ReflectionHelloWorld {
	public static void main(String[] args){
		Foo f = new Foo();

		Method method;
		try {
			method = f.getClass().getMethod("print", new Class<?>[0]);
			method.invoke(f);
		} catch (Exception e) {
			e.printStackTrace();
		}			
	}
}

class Foo {
	public void print() {
		System.out.println("abc");
	}
}

 Example 3: Create object from Class instance

public class ReflectionHelloWorld {
	public static void main(String[] args){
		//create instance of "Class"
		Class<?> c = null;
		try{
			c=Class.forName("myreflection.Foo");
		}catch(Exception e){
			e.printStackTrace();
		}

		//create instance of "Foo"
		Foo f = null;

		try {
			f = (Foo) c.newInstance();
		} catch (Exception e) {
			e.printStackTrace();
		}	

		f.print();
	}
}

class Foo {
	public void print() {
		System.out.println("abc");
	}
}

 Example 4: Get constructor and create instance

package myreflection;

import java.lang.reflect.Constructor;

public class ReflectionHelloWorld {
	public static void main(String[] args){
		//create instance of "Class"
		Class<?> c = null;
		try{
			c=Class.forName("myreflection.Foo");
		}catch(Exception e){
			e.printStackTrace();
		}

		//create instance of "Foo"
		Foo f1 = null;
		Foo f2 = null;

		//get all constructors
		Constructor<?> cons[] = c.getConstructors();

		try {
			f1 = (Foo) cons[0].newInstance();
			f2 = (Foo) cons[1].newInstance("abc");
		} catch (Exception e) {
			e.printStackTrace();
		}	

		f1.print();
		f2.print();
	}
}

class Foo {
	String s; 

	public Foo(){}

	public Foo(String s){
		this.s=s;
	}

	public void print() {
		System.out.println(s);
	}
}

 Example 5: Change array size though reflection

package myreflection;

import java.lang.reflect.Array;

public class ReflectionHelloWorld {
	public static void main(String[] args) {
		int[] intArray = { 1, 2, 3, 4, 5 };
		int[] newIntArray = (int[]) changeArraySize(intArray, 10);
		print(newIntArray);

		String[] atr = { "a", "b", "c", "d", "e" };
		String[] str1 = (String[]) changeArraySize(atr, 10);
		print(str1);
	}

	// change array size
	public static Object changeArraySize(Object obj, int len) {
		Class<?> arr = obj.getClass().getComponentType();
		Object newArray = Array.newInstance(arr, len);

		//do array copy
		int co = Array.getLength(obj);
		System.arraycopy(obj, 0, newArray, 0, co);
		return newArray;
	}

	// print
	public static void print(Object obj) {
		Class<?> c = obj.getClass();
		if (!c.isArray()) {
			return;
		}

		System.out.println("
Array length: " + Array.getLength(obj));

		for (int i = 0; i < Array.getLength(obj); i++) {
			System.out.print(Array.get(obj, i) + " ");
		}
	}
}