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 APIimport 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(";");  }}