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);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(";"); }}