示例对象
public class People { private String name; private String address; public People() { } public People(String name, String address) { this.name = name; this.address = address; } public String getName() { return name; } public void setName(String name) { this.name = name; } public String getAddress() { return address; } public void setAddress(String address) { this.address = address; }}
构造器创建对象
People people = new People("lili", "china");
利用class的反射机制
Class peopleClass = People.class; //获得People的Class实例对象People people = (People)peopleClass.newInstance(); //Class clazz = Class.forName("People");People people = (People)clazz.newInstance();
这种方式适用于有无参构造器的类
利用序列化的方式构造实例对象
public class CreateFour implements Serializable { public static void main(String args[]) { CreateFour fCreateFour = new CreateFour(); ObjectOutputStream objectStream; try { objectStream = new ObjectOutputStream(new FileOutputStream("res/obj.txt")); objectStream.writeObject(fCreateFour); ObjectInputStream objectInputStream = new ObjectInputStream(new FileInputStream("res/obj.txt")); CreateFour cloneFour = (CreateFour) objectInputStream.readObject(); } catch (FileNotFoundException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (IOException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (ClassNotFoundException e) { // TODO Auto-generated catch block e.printStackTrace(); } } }
这种方式前提是该类必须要实现Serializable
利用Object的clone方法
public class CreateFour implements Cloneable {
public static void main(String args[]) {
CreateFour f = new CreateFour();
try {
CreateFour cloneObject = (CreateFour) f.clone();
} catch (CloneNotSupportedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
该类必须实现Cloneable接口，不然会抛出异常CloneNotSupportedException
使用Objenesis来实例化对象
Objenesis objenesis = new ObjenesisStd();ObjectInstantiator instantiator = objenesis.getInstantiatorOf(People.class);People people = (People) instantiator.newInstance();
使用场合
Java已经支持使用Class.newInstance()动态实例化类的实例。但是类必须拥有一个合适的构造器。有很多场景下不能使用这种方式实例化类，比如：
- 构造器需要参数
- 构造器有side effects
- 构造器会抛异常
因此，在类库中经常会有类必须拥有一个默认构造器的限制。Objenesis通过绕开对象实例构造器来克服这个限制。
典型应用
- 序列化，远程调用和持久化 -对象需要实例化并存储为到一个特殊的状态，而没有调用代码。
- 代理，AOP库和Mock对象 -类可以被子类继承而子类不用担心父类的构造器
- 容器框架 -对象可以以非标准的方式被动态实例化。
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