Java 学习笔记：1，类引导，反射机制下的对象创建

1， 类引导，反射机制下的对象创建
类引导最终会字节流传给native 的java.lang.ClassLoader.defineClass1方法。
Class的 newInstance 方法 创建对象实例的时候，会调用无参的构造函数，如果类中没有 无参数的构造函数，JVM 会抛出java.lang.InstantiationException异常，无法进行实例化。
针对有参数的构造函数的类，可以调用class的getDeclaredMethod和Constructor 来构造这一个class的对象。

如果无参的构造函数是private类型，JVM调用该类时，会报private权限的错误。将无参的构造函数设为public类型可以解决该问题。

URL url1 = new URL("file:/E:/Server/vsmessages_fortest.jar");
			URLClassLoader myClassLoader = new URLClassLoader( new  URL[]   { url1 } );
			Class myClass  =  myClassLoader.loadClass("cn.com.*.messageagent.vo.MessageVO");
			Method m01 = myClass.getDeclaredMethod("setClientId", String.class);
			Method m02 = myClass.getDeclaredMethod("getClientId");


			Constructor ctor = myClass.getConstructor(String.class);

		    ctor.setAccessible(true);
		    Object targetObj1 = ctor.newInstance("1234567");

//			m01.invoke(targetObj1, "123456");
			String clientId = (String) m02.invoke(targetObj1);
			log.info("clientId:" + clientId);

备注：
Java in a Nutshell这本书的第12章介绍了与class的反射机制有关的基本方法。

Example 12.1: Obtaining Class and Member Information with the Reflection API

import java.lang.reflect.*;
/** A program that displays a class synopsis for the named class. */
public class ShowClass {
  /** The main method.  Print info about the named class. */
  public static void main(String[] args) throws ClassNotFoundException {
    Class c = Class.forName(args[0]);
    print_class(c);
  }
  /** Display the modifiers, name, superclass, and interfaces of a class
   *  or interface. Then go and list all constructors, fields, and methods. */
  public static void print_class(Class c)
  {
    // Print modifiers, type (class or interface), name, and superclass.
    if (c.isInterface()) {
      // The modifiers will include the "interface" keyword here...
      System.out.print(Modifier.toString(c.getModifiers()) +  c.getName());
    }
    else
      System.out.print(Modifier.toString(c.getModifiers()) + " class " +
                       c.getName() +
                       " extends " + c.getSuperclass().getName());
    // Print interfaces or super-interfaces of the class or interface.
    Class[] interfaces = c.getInterfaces();
    if ((interfaces != null) && (interfaces.length > 0)) {
      if (c.isInterface()) System.out.println(" extends ");
      else System.out.print(" implements ");
      for(int i = 0; i < interfaces.length; i++) {
        if (i > 0) System.out.print(", ");
        System.out.print(interfaces[i].getName());
      }
    }
    System.out.println(" {");            // Begin class member listing.
    // Now look up and display the members of the class.
    System.out.println(" // Constructors");
    Constructor[] constructors = c.getDeclaredConstructors();
    for(int i = 0; i < constructors.length; i++)      // Display constructors.
      print_method_or_constructor(constructors[i]);
    System.out.println(" // Fields");
    Field[] fields = c.getDeclaredFields();           // Look up fields.
    for(int i = 0; i < fields.length; i++)            // Display them.
      print_field(fields[i]);
    System.out.println(" // Methods");
    Method[] methods = c.getDeclaredMethods();        // Look up methods.
    for(int i = 0; i < methods.length; i++)           // Display them.
      print_method_or_constructor(methods[i]);
    System.out.println("}");             // End class member listing.
  }
  /** Return the name of an interface or primitive type, handling arrays. */
  public static String typename(Class t) {
    String brackets = "";
    while(t.isArray()) {
      brackets += "[]";
      t = t.getComponentType();
    }
    return t.getName() + brackets;
  }
  /** Return a string version of modifiers, handling spaces nicely. */
  public static String modifiers(int m) {
    if (m == 0) return "";
    else return Modifier.toString(m) + " ";
  }
  /** Print the modifiers, type, and name of a field. */
  public static void print_field(Field f) {
    System.out.println("  " +
                       modifiers(f.getModifiers()) +
                       typename(f.getType()) + " " + f.getName() + ";");
  }
  /** Print the modifiers, return type, name, parameter types, and exception
   *  type of a method or constructor.  Note the use of the Member interface
   *  to allow this method to work with both Method and Constructor objects. */
  public static void print_method_or_constructor(Member member) {
    Class returntype=null, parameters[], exceptions[];
    if (member instanceof Method) {
      Method m = (Method) member;
      returntype = m.getReturnType();
      parameters = m.getParameterTypes();
      exceptions = m.getExceptionTypes();
    } else {
      Constructor c = (Constructor) member;
      parameters = c.getParameterTypes();
      exceptions = c.getExceptionTypes();
    }
    System.out.print("  " + modifiers(member.getModifiers()) +
                     ((returntype!=null)? typename(returntype)+" " : "") +
                     member.getName() + "(");
    for(int i = 0; i < parameters.length; i++) {
      if (i > 0) System.out.print(", ");
      System.out.print(typename(parameters[i]));
    }
    System.out.print(")");
    if (exceptions.length > 0) System.out.print(" throws ");
    for(int i = 0; i < exceptions.length; i++) {
      if (i > 0) System.out.print(", ");
      System.out.print(typename(exceptions[i]));
    }
    System.out.println(";");
  }
}