利用反射分析类的能力

检查类的结构

反射机制最重要的内容 - 检查类的结构.

Class	Description
AccessibleObject	The AccessibleObject class is the base class for Field, Method, and Constructor objects (known as reflected objects).
Array	The Array class provides static methods to dynamically create and access Java arrays.
Constructor\	Constructor provides information about, and access to, a single constructor for a class.
Executable	A shared superclass for the common functionality of Method and Constructor.
Field	A Field provides information about, and dynamic access to, a single field of a class or an interface.
Method	A Method provides information about, and access to, a single method on a class or interface.
Modifier	The Modifier class provides static methods and constants to decode class and member access modifiers.
Parameter	Information about method parameters.
Proxy	Proxy provides static methods for creating objects that act like instances of interfaces but allow for customized method invocation.
ReflectPermission	The Permission class for reflective operations.

在 java.lang.reflect 包中有三个类 Field, Method 和 Constructor 分别用于描述类的域,方法和构造器.这三个类都有一个叫做 getName 的方法,用来返回项目的名称.Fleld 类有一个 getType 方法,用来返回描述域所属类型的 Class 对象.Method 和 Constructor 类有能够报告参数类型的方法,Method 类还有一个可以报告返回类型的方法.这三个类还有一个叫做 getModifiers 的方法,它将返回一个整型数值,用不同的位开关描述 public 和 static 这样的修饰符使用状况.另外,还可以利用 java.lang.reflect 包中的 Modifier 类的静态方法分析 getModifiers 返回的整型数值.例如,可以使用 Modifier 类中的 isPublic , isPrivate 或 isFinal判断方法或构造器是否是 public , private 或 final.我们需要做的全部工作就是调用 Modifier 类的相应方法,并对返回的整型数值进行分析,另外,还可以利用 Modifier.toString 方法将修饰符打印出来.

Class 类中的 getFields , getMethods 和 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.Class 类的 getDeclareFields,getDeclareMethods 和 getDeclaredConstructors 方法将分别返回类中声明的全部域,方法和构造器,其中包括私有和受保护成员,但不包括超类的成员 .

下面代码显示了如何打印一个类的全部信息的方法.这个程序将提醒用户输入类名,然后输出类中所有的方法和构造器的签名,以及全部域名.假如输入

java.util.Date

程序输出:

publicclass java.util.Date
{
   public java.util.Date(java.lang.String);
   public java.util.Date(int, int, int, int, int, int);
   public java.util.Date(int, int, int, int, int);
   public java.util.Date();
   public java.util.Date(long);
   public java.util.Date(int, int, int);

   public boolean equals(java.lang.Object);
   public java.lang.String toString();
   public int hashCode();
   public java.lang.Object clone();
   public volatile int compareTo(java.lang.Object);
   public int compareTo(java.util.Date);
   public static java.util.Date from(java.time.Instant);
   private void readObject(java.io.ObjectInputStream);
   private void writeObject(java.io.ObjectOutputStream);
   private final sun.util.calendar.BaseCalendar$Date normalize(sun.util.calendar.BaseCalendar$Date);
   private final sun.util.calendar.BaseCalendar$Date normalize();
   public boolean before(java.util.Date);
   public boolean after(java.util.Date);
   public static long parse(java.lang.String);
   public int getSeconds();
   public java.time.Instant toInstant();
   public static long UTC(int, int, int, int, int, int);
   public int getYear();
   public long getTime();
   public void setTime(long);
   static final long getMillisOf(java.util.Date);
   public void setDate(int);
   public int getMonth();
   private final sun.util.calendar.BaseCalendar$Date getCalendarDate();
   public void setHours(int);
   public int getHours();
   public int getMinutes();
   public void setMonth(int);
   public void setMinutes(int);
   public void setSeconds(int);
   public void setYear(int);
   private static final sun.util.calendar.BaseCalendar getCalendarSystem(long);
   private static final sun.util.calendar.BaseCalendar getCalendarSystem(int);
   private static final sun.util.calendar.BaseCalendar getCalendarSystem(sun.util.calendar.BaseCalendar$Date);
   private final long getTimeImpl();
   private static final java.lang.StringBuilder convertToAbbr(java.lang.StringBuilder, java.lang.String);
   private static final synchronized sun.util.calendar.BaseCalendar getJulianCalendar();
   public int getDate();
   public int getDay();
   public java.lang.String toLocaleString();
   public java.lang.String toGMTString();
   public int getTimezoneOffset();

  public java.util.Date(java.lang.String);
  public java.util.Date(int, int, int, int, int, int);
  public java.util.Date(int, int, int, int, int);
  public java.util.Date();
  public java.util.Date(long);
  public java.util.Date(int, int, int);
}

Process finished with exit code 0

代码

package reflection;

import java.util.*;
import java.lang.reflect.*;

public class ReflectionTest {
    public static void main(String[] args) {
        // read class name from command line args or user input
        String name;
        if(args.length > 0) name = args[0];
        else
        {
            Scanner in = new Scanner(System.in);
            System.out.println("输入一个类的名称 (例如 java.util.Date): ");
            name = in.next();
        }

        try {
            // Print class name and superclass name (if != Object)
            Class cl = Class.forName(name);
            Class supercl = cl.getSuperclass();
            String modifiers = Modifier.toString(cl.getModifiers());
            if(modifiers.length() > 0) System.out.print(modifiers + "");
            System.out.print("class " + name);
            if(supercl != null && supercl != Object.class) System.out.printf(" extends " + supercl.getName());
            System.out.printf("\n{\n");
            printConstructors(cl);
            System.out.println();
            printMethods(cl);
            System.out.println();
            printFields(cl);
            System.out.println("}");
        }
        catch (ClassNotFoundException e)
        {
            e.printStackTrace();
        }
        System.exit(0);
    }

    /**
     * Prints all constructors of a class
     * @param cl a class
     */
    private static void printFields(Class cl) {
        Constructor[] constructors = cl.getDeclaredConstructors();

        for (Constructor c : constructors)
        {
            String name = c.getName();
            System.out.printf("  ");
            String modifiers = Modifier.toString(c.getModifiers());
            if(modifiers.length() > 0) System.out.printf(modifiers + " ");
            System.out.printf(name + "(");

            // print parameter types
            Class[] paramTypes = c.getParameterTypes();
            for (int j = 0; j < paramTypes.length; j++)
            {
                if(j > 0) System.out.printf(", ");
                System.out.printf(paramTypes[j].getName());
            }
            System.out.println(");");
        }
    }

    /**
     * Prints all methods of a class
     * @param cl a class
     */
    public static void printMethods(Class cl)
    {
        Method[] methods = cl.getDeclaredMethods();

        for (Method m : methods)
        {
            Class retType = m.getReturnType();
            String name = m.getName();

            System.out.print("   ");
            // print modifiers, return type and method name
            String modifiers = Modifier.toString(m.getModifiers());
            if (modifiers.length() > 0) System.out.print(modifiers + " ");
            System.out.print(retType.getName() + " " + name + "(");

            // print parameter types
            Class[] paramTypes = m.getParameterTypes();
            for (int j = 0; j < paramTypes.length; j++)
            {
                if (j > 0) System.out.print(", ");
                System.out.print(paramTypes[j].getName());
            }
            System.out.println(");");
        }
    }

    /**
     * Prints all constructors of a class
     * @param cl a class
     */
    public static void printConstructors(Class cl)
    {
        Constructor[] constructors = cl.getDeclaredConstructors();

        for (Constructor c : constructors)
        {
            String name = c.getName();
            System.out.print("   ");
            String modifiers = Modifier.toString(c.getModifiers());
            if (modifiers.length() > 0) System.out.print(modifiers + " ");
            System.out.print(name + "(");

            // print parameter types
            Class[] paramTypes = c.getParameterTypes();
            for (int j = 0; j < paramTypes.length; j++)
            {
                if (j > 0) System.out.print(", ");
                System.out.print(paramTypes[j].getName());
            }
            System.out.println(");");
        }
    }
}