反射初步

在面向对象的世界里，万物皆对象。类是对象，类是java.lang.Class类的实例对象。另外class类只有java虚拟机才能new出来。任何一个类都是Class。
类的实例对象。这实例对象有三种表达方式：

Class s1=Class.forName("com.lmr.invoke.Student");Class s2=Student.class;Student s=new Student();Class s3=s.getClass();

Class.forName(类的全称);该方法不仅表示了类的类型，还代表了动态加载类。编译时刻加载类是静态加载、运行时刻加载类是动态加载类。

可以利用==运算符实现两个类对象的比较

if(e.getClass()==xx.class){}

newInstance()可以用来快速创建一个类的实例,前提是该类必须存在一个无参构造方法.

e.getClass.newInstance()

eg1：

private static void initClass() throws ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchMethodException, SecurityException, IllegalArgumentException, InvocationTargetException {        // TODO Auto-generated method stub        //三种实例方式        Class s1=Class.forName("com.lmr.invoke.Student");        Class s2=Student.class;        Student s=new Student(1,"1",1);        Class s3=s.getClass();        System.out.println(s1.getName());        System.out.println(s1.getSimpleName());        System.out.println(s2.getName());        System.out.println(s3.getName());        Student stu1=(Student) s1.newInstance();//通过类的类型创建该类的实例对象 ，前提是该类必须存在一个无参构造方法        Constructor con=s2.getConstructor(int.class,String.class,int.class);//取得指定参数类型的构造方法        Student stu2=(Student) con.newInstance(1,"name",12);        System.out.println(stu1.toString());        System.out.println(stu2.toString());    }

结果：

com.lmr.invoke.StudentStudentcom.lmr.invoke.Studentcom.lmr.invoke.StudentStudent [id=0, name=null, age=0]Student [id=1, name=name, age=12]

通过反射来获取类的结构。
Class类中的getFields()、getMethods()、getConstructors()方法将分别返回类提供的public域、方法和构造器数组，其中包括超类的公有成员。
eg2：

private static void initClassInfo() {        // TODO Auto-generated method stub//      Class c=Student.class;        Class c=Math.class;        System.out.println(c.getName());        System.out.println(c.getSimpleName());//      System.out.println(c.getComponentType());//      Field[] fields=c.getFields();//获取public类型的成员变量        Field[] fields=c.getDeclaredFields();//成员变量信息,获取全部        System.out.println("----------成员变量----------");        for(Field f:fields){            int mod=f.getModifiers();            String modtype=Modifier.toString(mod);//方法的属性            System.out.println(modtype+" "+f.getType().getSimpleName()+" "+f.getName());        }        Constructor[] constructors=c.getDeclaredConstructors();//获取构造函数信息        System.out.println("----------构造函数----------");        for(Constructor con:constructors){            String name=con.getName();//方法名            Class[] paramterType=con.getParameterTypes();//方法中参数类型            int mod=con.getModifiers();            String modtype=Modifier.toString(mod);//方法的属性            System.out.print(modtype+" "+name+" (");            for(Class p:paramterType){                System.out.print(p.getSimpleName()+" ");            }            System.out.println(")");        }        Method[] methods=c.getMethods();//获取函数方法信息        System.out.println("-----------方法-----------");        for(Method m:methods){            String name=m.getName();//方法名            Class returnType=m.getReturnType();//方法返回类型            Class[] paramterType=m.getParameterTypes();//方法中参数类型            int mod=m.getModifiers();            String modtype=Modifier.toString(mod);//方法的属性            System.out.print(modtype+" "+returnType.getSimpleName()+" "+name+" (");            for(Class p:paramterType){                System.out.print(p.getSimpleName()+" ");            }            System.out.println(")");        }    }

结果：

java.lang.MathMath----------成员变量----------public static final double Epublic static final double PIprivate static long negativeZeroFloatBitsprivate static long negativeZeroDoubleBitsstatic double twoToTheDoubleScaleUpstatic double twoToTheDoubleScaleDownstatic final boolean $assertionsDisabled----------构造函数----------private java.lang.Math ()-----------方法-----------public static int abs (int )public static double abs (double )public static float abs (float )public static long abs (long )public static double sin (double )public static double cos (double )public static double tan (double )public static double atan2 (double double )public static double sqrt (double )public static double log (double )public static double log10 (double )public static double pow (double double )public static double exp (double )public static long min (long long )public static double min (double double )public static float min (float float )public static int min (int int )public static double max (double double )public static float max (float float )public static long max (long long )public static int max (int int )public static long addExact (long long )public static int addExact (int int )public static int decrementExact (int )public static long decrementExact (long )public static long incrementExact (long )public static int incrementExact (int )public static long multiplyExact (long long )public static int multiplyExact (int int )public static int negateExact (int )public static long negateExact (long )public static int subtractExact (int int )public static long subtractExact (long long )public static double scalb (double int )public static float scalb (float int )public static double copySign (double double )public static float copySign (float float )public static int getExponent (float )public static int getExponent (double )public static double signum (double )public static float signum (float )public static double IEEEremainder (double double )public static double acos (double )public static double asin (double )public static double atan (double )public static double cbrt (double )public static double ceil (double )public static double cosh (double )public static double expm1 (double )public static double floor (double )public static long floorDiv (long long )public static int floorDiv (int int )public static int floorMod (int int )public static long floorMod (long long )public static double hypot (double double )public static double log1p (double )public static float nextAfter (float double )public static double nextAfter (double double )public static double nextDown (double )public static float nextDown (float )public static double nextUp (double )public static float nextUp (float )public static double random ()public static double rint (double )public static int round (float )public static long round (double )public static double sinh (double )public static double tanh (double )public static double toDegrees (double )public static int toIntExact (long )public static double toRadians (double )public static double ulp (double )public static float ulp (float )public final void wait ()public final void wait (long int )public final native void wait (long )public boolean equals (Object )public String toString ()public native int hashCode ()public final native Class getClass ()public final native void notify ()public final native void notifyAll ()

获取一个方法：需要获取方法的名称和方法的参数才能决定一个方法。

Class[] paramtype={int.class, int.class};Method m=calculateclass.getMethod("add", paramtype);Method m=calculateclass.getMethod("add", int.class, int.class);

方法的反射操作：method.invoke(对象，参数列表);

eg1：

public static void InvokeOne(){    try {        Class calculateclass=Class.forName("com.lmr.invoke.Calculate");        Class[] paramtype={int.class, int.class};        Method m=calculateclass.getMethod("add", paramtype);//          Method m=calculateclass.getMethod("add", int.class, int.class);        m.invoke(calculateclass.newInstance(), 1, 2);//method.invoke(对象，参数列表);    } catch (ClassNotFoundException e) {        // TODO Auto-generated catch block        e.printStackTrace();    } catch (NoSuchMethodException e) {        // TODO Auto-generated catch block        e.printStackTrace();    } catch (SecurityException e) {        // TODO Auto-generated catch block        e.printStackTrace();    } catch (IllegalAccessException e) {        // TODO Auto-generated catch block        e.printStackTrace();    } catch (InvocationTargetException e) {        // TODO Auto-generated catch block        e.printStackTrace();    } catch (IllegalArgumentException e) {        // TODO Auto-generated catch block        e.printStackTrace();    } catch (InstantiationException e) {        // TODO Auto-generated catch block        e.printStackTrace();    }}public class Calculate {    public void add(int a,int b){        System.out.println(a+b);    }    public void subtract(int a,int b){        System.out.println(a-b);    }    public void multiply(int a,int b){        System.out.println(a*b);    }    public void divide(int a,int b){        if(b!=0){            System.out.println(a/b);        }    }}

结果：打印出1+2的结果，3

Java的反射机制的实现要借助于4个类：class，Constructor，Field，Method;
class－类对象
Constructor－类的构造器对象
Field－类的属性对象
Method－类的方法对象

反射的操作都是在运行时进行的

private static void InvokeTwo() {        // TODO Auto-generated method stub    try {        Class studentclass=Class.forName("com.lmr.invoke.Student");        Object stuobj=studentclass.newInstance();        Method setStuName=studentclass.getMethod("setName", String.class);        setStuName.invoke(stuobj, "Jack");//          PropertyDescriptor pd=new PropertyDescriptor("name", studentclass);//通过属性名来获取对应的存储器，从而获取其get，set方法//          Method setStuName=pd.getWriteMethod();        Method toStuString=studentclass.getMethod("toString");        System.out.println(toStuString.invoke(stuobj));        Field name=studentclass.getDeclaredField("name");        name.setAccessible(true);//由于name是一个私有域，所有须解除封装        name.set(stuobj, "Jane");        System.out.println(toStuString.invoke(stuobj));    } catch (ClassNotFoundException e) {        // TODO Auto-generated catch block        e.printStackTrace();    } catch (InstantiationException e) {        // TODO Auto-generated catch block        e.printStackTrace();    } catch (IllegalAccessException e) {        // TODO Auto-generated catch block        e.printStackTrace();    } catch (NoSuchMethodException e) {        // TODO Auto-generated catch block        e.printStackTrace();    } catch (SecurityException e) {        // TODO Auto-generated catch block        e.printStackTrace();    } catch (IllegalArgumentException e) {        // TODO Auto-generated catch block        e.printStackTrace();    } catch (InvocationTargetException e) {        // TODO Auto-generated catch block        e.printStackTrace();    } catch (NoSuchFieldException e) {        // TODO Auto-generated catch block        e.printStackTrace();    }}

结果：

Student [id=0, name=Jack, age=0]Student [id=0, name=Jane, age=0]