Java反射机制实例代码分享

String s = "ziwenxie";
Class c = s.getClass();

Class c = Class.forName("java.lang.String");

Class c = String.class;

package typeinfo.interfacea;
public interface A { void f(); }

package typeinfo.packageaccess;
import typeinfo.interfacea.A;
class C implements A {
  public void f() { System.out.println("public C.f()"); }
  public void g() { System.out.println("public C.g()"); }
  protected void v () { System.out.println("protected C.v()"); }
  void u() { System.out.println("package C.u()"); }
  private void w() { System.out.println("private C.w()"); }
}
public class HiddenC {
  public static A makeA() { return new C(); }
}

package typeinfo;
import typeinfo.interfacea.A;
import typeinfo.packageaccess.HiddenC;
import java.lang.reflect.Method;
public class HiddenImplementation {
  public static void main(String[] args) throws Exception {
    A a = HiddenC.makeA();
    a.f();
    System.out.println(a.getClass().getName());
    // Oops! Reflection still allows us to call g():
    callHiddenMethod(a, "g");
    // And even methods that are less accessible!
    callHiddenMethod(a, "u");
    callHiddenMethod(a, "v");
    callHiddenMethod(a, "w");
  }
  static void callHiddenMethod(Object a, String methodName) throws Exception {
    Method g = a.getClass().getDeclaredMethod(methodName);
    g.setAccessible(true);
    g.invoke(a);
  }
}

public C.f()
typeinfo.packageaccess.C
public C.g()
package C.u()
protected C.v()
private C.w()

public class Pet extends Individual {
  public Pet(String name) { super(name); }
  public Pet() { super(); }
}
public class Cat extends Pet {
  public Cat(String name) { super(name); }
  public Cat() { super(); }
}
public class Dog extends Pet {
  public Dog(String name) { super(name); }
  public Dog() { super(); }
}
public class EgyptianMau extends Cat {
  public EgyptianMau(String name) { super(name); }
  public EgyptianMau() { super(); }
}
public class Mutt extends Dog {
  public Mutt(String name) { super(name); }
  public Mutt() { super(); }
}

public class Individual implements Comparable {
  private static long counter = 0;
  private final long id = counter++;
  private String name; // name is optional
  public Individual(String name) { this.name = name; }
  public Individual() {}
  public String toString() {
    return getClass().getSimpleName() + (name == null ? "" : " " + name);
  }
  public long id() { return id; }
  public boolean equals(Object o) {
    return o instanceof Individual && id == ((Individual)o).id;
  }
  public int hashCode() {
    int result = 17;
    if (name != null) {
      result = 37 * result + name.hashCode();
    }
    result = 37 * result + (int) id;
    return result;
  }
  public int compareTo(Individual arg) {
    // Compare by class name first:
    String first = getClass().getSimpleName();
    String argFirst = arg.getClass().getSimpleName();
    int firstCompare = first.compareTo(argFirst);
    if (firstCompare != 0) {
      return firstCompare;
    }
    if (name != null && arg.name != null) {
      int secendCompare = name.compareTo(arg.name);
      if (secendCompare != 0) {
        return secendCompare;
      }
    }
    return (arg.id < id ? -1 : (arg.id == id ? 0 : 1));
  }
}

public abstract class PetCreator {
  private Random rand = new Random(47);
  // The List of the different getTypes of Pet to create:
  public abstract List> getTypes();
  public Pet randomPet() {
    // Create one random Pet
    int n = rand.nextInt(getTypes().size());
    try {
      return getTypes().get(n).newInstance();
    } catch (InstantiationException e) {
      throw new RuntimeException(e);
    } catch (IllegalAccessException e) {
      throw new RuntimeException(e);
    }
  }
  public Pet[] createArray(int size) {
    Pet[] result = new Pet[size];
    for (int i = 0; i < size; i++) {
      result[i] = randomPet();
    }
    return result;
  }
  public ArrayList arrayList(int size) {
    ArrayList result = new ArrayList();
    Collections.addAll(result, createArray(size));
    return result;
  }
}

public class LiteralPetCreator extends PetCreator {
  @SuppressWarnings("unchecked")
  public static final List> allTypes = Collections.unmodifiableList(
    Arrays.asList(Pet.class, Dog.class, Cat.class, Mutt.class, EgyptianMau.class));
  private static final List> types = allTypes.subList(
    allTypes.indexOf(Mutt.class), allTypes.size());
  public List> getTypes() {
    return types;
  }
}

public class TypeCounter extends HashMap, Integer> {
  private Class baseType;
  public TypeCounter(Class baseType) {
    this.baseType = baseType;
  }
  public void count(Object obj) {
    Class type = obj.getClass();
    if (!baseType.isAssignableFrom(type)) {
      throw new RuntimeException(
        obj + " incorrect type " + type + ", should be type or subtype of " + baseType);
    }
    countClass(type);
  }
  private void countClass(Class type) {
    Integer quantity = get(type);
    put(type, quantity == null ? 1 : quantity + 1);
    Class superClass = type.getSuperclass();
    if (superClass != null && baseType.isAssignableFrom(superClass)) {
      countClass(superClass);
    }
  }
  @Override
  public String toString() {
    StringBuilder result = new StringBuilder("{");
    for (Map.Entry, Integer> pair : entrySet()) {
      result.append(pair.getKey().getSimpleName());
      result.append("=");
      result.append(pair.getValue());
      result.append(", ");
    }
    result.delete(result.length() - 2, result.length());
    result.append("} ");
    return result.toString();
  }
}