java-reflection

Java reflection and introspector.

Reflection

1. Introduce
   Class descriptor
   Descripe features of class
   Class DOES NOT rewrite hashCode() (returns position in memory)

   // Persion is a class
   // Student is a class whitch extends Persion
   public void testClass(){
       Persion p = new Persion();
       Student s = new Student();
       Class class1 = p.getClass();
       Class class2 = Persion.class;
       
       System.out.println(p instanceof Persion)            //true
       System.out.println(p.getClass() == Persion.class)   //true
       System.out.println(s instanceof Persion)            //true
       System.out.println(s.getClass() == Persion.class)   //false
       
       System.out.println(class1 == class2); // true
       System.out.println(class1.hashCode() == class2.hashCode()); // true (position in memory)
       System.out.println(class1.getName()); // full name of Persion including package path
   }

2. Method, Field

   // Person class
   class Person {
       private String name；
       public void setName(String name) {
           this.name = name;
       }
       public String getName() {
           return this.name;
       }
       
       private void talk() {
           System.out.println("This is a private methed!");
       }
   }
   
   public void testNewInstance() throw Exception {
       /***Create Object Dynamically***/
       // Load class, return Class object.
       Class clazz = Class.forName("nan.javalearn.reflection.Persion");
       // Make object by Class.
       Object obj = clazz.newInstance();
       
       /***Method***/
       // Get `setName()` of Person
       Method m1 = clazz.getDeclareMethod("setName", String.class);
       // Call `setName()`
       m1.invoke(obj, "tom");
       
       // Get `getName()` of Person
       Method m2 = clazz.getDeclareMethod("getName");
       Object ret = m2.invoke(obj);
       System.out.println(ret);  // tom
       
       // Access private method
       Method mp = clazz.getDeclareMethod("talk");
       mp.setAccessible(true); // Set accessible.
       mp.invoke(obj);   // Output: This is a private method!
       
       // diff `getDeclareMethods()` and `getMethods()`
       Method[] ms = clazz.getMethods();
       for (Method mm : ms) {
           Systom.out.println(mm); // All methods including that extends from super class.
       }
       ms = clazz.getDeclareMethods();
       for (Method mm : ms) {
           System.out.println(mm); // Methods declared in this class.
       }
       
       // Get all available methods start with "get".
       ms = clazz.getMethods();
       for (Method mm : ms) {
           String fname = mm.getName();
           Class[] pytpes = mm.getParameterTypes();
           if (fname.startsWith("get") && (ptypes == null || ptypes.length == 0)) {
               int mod = mm.getModifiers();  // Get modifiers of this method.
               System.out.println(Modifer.isPublic(mod));        // If it is a public method.
               ret = mm.invoke(obj);
               System.out.println(fname + " = " + ret);
           }
       }
       
       /***Field***/
       Field f = clazz.getDeclareField("name");
       f.setAccessible(true);
       ret = f.get(obj);
       System.out.println(ret);  // Output: tom.
       f.set(obj, "jerry");  // set `name` to `jerry`.
   }

3. Constructor

   public class Person2{
       private String name;
       public Person2(String name) {
           this.name = name;
       }
   }
   
   public void testConstructor() throw Exception {
       Class clazz = Class.forName("nan.javalearn.reflect.Person2");
       Constructor cs = clazz.getDeclaredConstructor(String.class);
       Object obj = cs.newInstance("tom");
       System.out.println(obj);
   }

4. Copy Properties

   • By Filed

     public class Person3{
         private String name;
         public Person3(String name) {
             this.name = name;
         }
     }
     
     public void propCopy(Person3 p1, Person3 p2){
         Class clazz = p1.getClass();
         Field[] fs = clazz.getDeclareFields();
         for(Field f : fs) {
             f.setAccessible(true);
             Object ret = f.get(a);
             f.set(b, ret);
         }
     }

   • By Method
     Full code can be found in github

     public static void copyProperty(Object a, Object b) {
         Method[] ms = a.getClass().getDeclaredMethods();
         Class bclazz = b.getClass();
         for (Method m : ms) {
             String mname = m.getName();
             Class[] paramTypes = m.getParameterTypes();
             Class retType = m.getReturnType();
         
             if (mname.startsWith("get")
                 && (paramTypes == null || paramTypes.length == 0)
                 && retType != void.class) {
                 String bmname = mname.replace("get", "set");
                 try {
                     Method bm = bclazz.getDeclaredMethod(bmname, retType);
                     Object retVal = m.invoke(a);
                     bm.invoke(b, retVal);
                 }  catch (Exception e) {
                     continue;
                 }
             }
         }
     }

Other examples

1. Proxy pattern
   full code
2. Build object from config file
   full code