Java 反射机制

1. Class与class
   假如把class看做是一张图纸的话，那根据图纸加工出来的零件就是一个个的instance（实例）。那你有没有想过，实际上一张具体的图纸也可以看过是某个类型的实例呢？每张图纸是不是都有标题？有作者？有创作日期？是不是存在一个“类”专门来描述它？答案是，有，就是Class！
   class是描述对象的，Class是用来描述类的。

2. 获取Class

MyObject mObj = new MyObject();
Class clazz = mObj.getClass();

Class clazz2 = Class.forName("com.jeremy.reflection.MyObject ");

3. 使用Class

• 实例化对象

Class clazz = Class.forName("com.jeremy.universal_android.reflection.MyObject");
//通过无参构造函数实例化对象
MyObject object = (MyObject) clazz.newInstance();

• 调用其他构造函数

public class MyObject {

    final String TAG = "MyObject";

    String name;
    int index;

    public MyObject() {
        this.name = "default";
        this.index = 0;
    }

    public MyObject(String str) {
        this.name = str;
        this.index = 0;
    }

    public MyObject(String str, int i) {
        this.name = str;
        this.index = i;
    }

    public void print() {
        LogTool.e(TAG, "name --> " + name + ",index --> " + index);
    }
  public void printWithParam(String str) {
      LogTool.e(TAG, "printWithParam --> " + str);
  }
}

MyObject object;
Class clazz = Class.forName("com.jeremy.universal_android.reflection.MyObject");
Constructor<?> constructors[] = clazz.getConstructors();
object = (MyObject) constructors[0].newInstance();
object.print();
object = (MyObject) constructors[1].newInstance("My name is Obj");
object.print();
object = (MyObject) constructors[2].newInstance("My name is Obj", 100);
object.print();

结果为

E/MyObject: name --> default,index --> 0
E/MyObject: name --> My name is Obj,index --> 0
E/MyObject: name --> My name is Obj,index --> 100

• 获取实现的接口

public class MyObject implements Interface1, Interface2 {
}

Class<?> interfaces[] = clazz.getInterfaces();
for (int i = 0; i < interfaces.length; i++) {
   LogTool.e(TAG, interfaces[i].getName());
}

E/MyObject: com.jeremy.universal_android.reflection.Interface1
E/MyObject: com.jeremy.universal_android.reflection.Interface2

• 获得父类

Class<?> parents = clazz.getSuperclass();
LogTool.e(TAG, parents.getName());

结果

E/MyObject: java.lang.Object

• 获得完整构造函数的签名

StringBuilder method;
Constructor<?> con;
Constructor<?> cons[] = clazz.getConstructors();
for (int i = 0; i < cons.length; i++) {
    method = new StringBuilder();
    con = cons[i];
//    获取修饰符
    int modifier = con.getModifiers();
    method.append(Modifier.toString(modifier));
//    获取构造函数名称
    method.append(" ").append(con.getName());
    method.append("(");
//    获取构造函数参数类型
    Class<?> params[] = con.getParameterTypes();
    for (int j = 0; j < params.length; ++j) {
        method.append(params[j].getName() + " arg" + i);
        if (j < params.length - 1) {
              method.append(",");
        }
    }
    method.append("){}");
    LogTool.e(TAG, method.toString());
}

结果

 E/MyObject: public com.jeremy.universal_android.reflection.MyObject(){}
 E/MyObject: public com.jeremy.universal_android.reflection.MyObject(java.lang.String arg1){}
 E/MyObject: public com.jeremy.universal_android.reflection.MyObject(java.lang.String arg2,int arg2){}

• 获得本类属性

 Field field = null;
StringBuilder fieldDes = null;
//获得本类属性
Field[] fields = clazz.getDeclaredFields();
//获得父类属性
//Field[] fields = clazz.getFields();
for (int i = 0; i < fields.length; i++) {
    fieldDes = new StringBuilder();
    field = fields[i];
    //    获取修饰符
    int modifier = field.getModifiers();
    fieldDes.append(Modifier.toString(modifier));

    Class<?> type = field.getType();
    fieldDes.append(" " + type.getName() + " " + field.getName());
    LogTool.e(TAG, fieldDes.toString());
}

结果

E/MyObject: public final java.lang.String TAG
E/MyObject: public java.lang.String name
E/MyObject: public int index

• 通过反射调用方法

//通过反射调用方法
Method method1 = clazz.getMethod("print");
method1.invoke(clazz.newInstance());
//调用有参数的方法
method1 = clazz.getMethod("printWithParam", String.class);
method1.invoke(clazz.newInstance(), "hello world");

结果

E/MyObject: name --> default,index --> 0
E/MyObject: printWithParam --> hello world