java 知识之 反射的使用

Class c1 = Person.class;

Person person = new Person();
Class c2 = person.getClass();

Class c3 = null;
   try {
       c3 = Class.forName("Person");
   } catch (ClassNotFoundException e) {
       e.printStackTrace();
   }

System.out.println(c1 == c2);//true
System.out.println(c1 == c3);//true
System.out.println(c2 == c3);//true

public class Object {
    
    // ...
     
    /**
    * Returns the runtime class of this {@code Object}. The returned
    * {@code Class} object is the object that is locked by {@code
    * static synchronized} methods of the represented class.
    *
    * <p><b>The actual result type is {@code Class<? extends |X|>}
    * where {@code |X|} is the erasure of the static type of the
    * expression on which {@code getClass} is called.</b> For
    * example, no cast is required in this code fragment:</p>
   
    * @return The {@code Class} object that represents the runtime
    *         class of this object.
    *         
*/
   public final native Class<?> getClass();
   
  // ...

public static Class<?> forName(String className)

public class ClassUtils {
    public static void getClassField(Object obj) {
            Class c = obj.getClass();
            Field[] fs = c.getDeclaredFields();// c.getMethods();
            for (Field f : fs) {
                Class fieldType = f.getType();
                System.out.println(fieldType.getName() + " " + f.getName());
            }
        }
}

ClassUtils.getClassField("reflect");

[C value
int hash
long serialVersionUID
[Ljava.io.ObjectStreamField; serialPersistentFields
java.util.Comparator CASE_INSENSITIVE_ORDER

public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence {
    
    private final char value[];
    private int hash; // Default to 0
    private static final long serialVersionUID = -6849794470754667710L;
    private static final ObjectStreamField[] serialPersistentFields =
            new ObjectStreamField[0];
            
    //...
     public static final Comparator<String> CASE_INSENSITIVE_ORDER
                                         = new CaseInsensitiveComparator();
    
    //...
}

public class ClassUtils {
    //...
    
    /**
     * 获取所有的成员方法
     *
     * @param obj
     */
    public static void getClassMethods(Object obj) {
        Class c = obj.getClass();
        System.out.println("类名为：" + c.getName());
        Method[] ms = c.getDeclaredMethods();
        for (Method m : ms) {
            Class returnType = m.getReturnType();
            //返回值的类类型，如String-->String.class
            System.out.print(returnType.getName() + " ");
            //方法名称
            System.out.print(m.getName() + "(");
            //获取参数--参数列表的类类型
            Class[] paramTypes = m.getParameterTypes();
            int paramsNum = paramTypes.length;
            Class paramType;
            for (int i = 0; i < paramsNum; i++) {
                paramType = paramTypes[i];
                if (i == 1 || (i == paramsNum - 1))
                    System.out.print(paramType.getName());
                else
                    System.out.print(paramType.getName() + " ,");
            }
            System.out.println(")");
        }
    }
    /**
     * 获取所有的构造函数
     *
     * @param obj
     */
    public static void printConMessage(Object obj) {
        Class c = obj.getClass();
        Constructor[] cs = c.getDeclaredConstructors();
        for (Constructor constructor : cs) {
            System.out.print(constructor.getName() + " (");
            Class[] paramTypes = constructor.getParameterTypes();
            int paramsNum = paramTypes.length;
            Class paramType;
            for (int i = 0; i < paramsNum; i++) {
                paramType = paramTypes[i];
                if (i == 1 || (i == paramsNum - 1))
                    System.out.print(paramType.getName());
                else
                    System.out.print(paramType.getName() + " ,");
            }
            System.out.println(")");
        }
    }
}

public class TestDemo {
    public static void main(String[] args) {
        //1.获取类信息
        A a = new A();
        Class aClass = a.getClass();
        //2.获取方法名称和参数
        try {
//            Method method = aClass.getMethod("print",new Class[]{int.class,int.class});
            Method method = aClass.getMethod("print", int.class, int.class);
            //方法返回结果：若无返回结果则o为null，否则为返回结果，这里返回为30
            Object o = method.invoke(a, 10, 20);
            System.out.println(o);//30
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
    }
}
//测试反射方法
class A {
    public int print(int a, int b) {
        System.out.println(a + b);
        return a + b;
    }
    public void print(String a, String b) {
        System.out.println(a.toUpperCase() + "-----" + b.toUpperCase());
    }
}

public class TestDemo04 {
    public static void main(String[] args) {
        ArrayList<String> l1 = new ArrayList<>();
        ArrayList l2 = new ArrayList();
        l1.add("java reflect");
//        datas.add(100);//报错
        Class c1 = l1.getClass();
        Class c2 = l2.getClass();
        System.out.println(c1 == c2);//true
        try {
            Method method = c2.getMethod("add", Object.class);
            method.invoke(l1, 100);
            System.out.println("数组大小："+l1.size());
            System.out.println("数组内容："+l1.toString());
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

true
数组大小：2
数组内容：[java reflect, 100]