使用JAXB创建不可变对象

Question

我正在使用JAXB从XSD文件创建Java对象.我正在创建不可变的包装器来隐藏JAXB生成的对象(之前我正在更新JAXB对象以实现不可变的接口并将接口返回到客户端.但是意识到改变自动生成的类是不好的,因此使用包装器)

目前我将这些不可变的包装器返回给客户端应用程序.是否有任何选项,以便自动生成的类将是不可变的,它将避免创建不可变包装器的额外工作.鼓励采取任何其他方法.

• 谢谢

Answer 1

从JSR-133(Java 1.5依赖)开始,您可以使用反射来设置未初始化的最终变量.所以你可以在私有构造函数中初始化为null,并且在没有任何XMLAdapter的情况下干净地使用JAXB + immutable.

来自https://test.kuali.org/svn/rice/sandbox/immutable-jaxb/的例子,来自对Blaise博客的评论http://blog.bdoughan.com/2010/12/jaxb-and-immutable -objects.html#带注释form_584069422380571931

package blog.immutable;

import javax.xml.bind.annotation.XmlAccessType;
import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlAttribute;
import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement(name="customer")
@XmlAccessorType(XmlAccessType.NONE)
public final class Customer {

    @XmlAttribute
    private final String name;

    @XmlElement
    private final Address address;

    @SuppressWarnings("unused")
    private Customer() {
        this(null, null);
    }

    public Customer(String name, Address address) {
        this.name = name;
        this.address = address;
    }

    public String getName() {
        return name;
    }

    public Address getAddress() {
        return address;
    }

}

Answer 2

您可以使用这些XJC编译器插件直接生成不可变类:

• https://github.com/sabomichal/immutable-xjc
• https://github.com/danielwegener/xjc-immutable-plugin

Answer 3

您可以在将 bean 返回给客户端之前为它们创建一个代理。您将需要javassist从类创建代理（可以直接使用 Java SE 从接口创建代理）。

然后，如果调用以“set”开头的方法，则可以抛出异常。

这是一个可重用的类，其方法可以包装“任何”POJO：

import java.lang.reflect.Method;

import javassist.util.proxy.MethodFilter;
import javassist.util.proxy.MethodHandler;
import javassist.util.proxy.Proxy;
import javassist.util.proxy.ProxyFactory;

public class Utils {

 public static <C> C createInmutableBean(Class<C> clazz, final C instance)
        throws InstantiationException, IllegalAccessException {
    if (!clazz.isAssignableFrom(instance.getClass())) {
        throw new IllegalArgumentException("given instance of class "
                + instance.getClass() + " is not a subclass of " + clazz);
    }
    ProxyFactory f = new ProxyFactory();
    f.setSuperclass(clazz);
    f.setFilter(new MethodFilter() {
        public boolean isHandled(Method m) {
            // ignore finalize()
            return !m.getName().equals("finalize");
        }
    });
    Class c = f.createClass();
    MethodHandler mi = new MethodHandler() {
        public Object invoke(Object self, Method m, Method proceed,
                Object[] args) throws Throwable {
            if (m.getName().startsWith("set")) {
                throw new RuntimeException("this bean is inmutable!");
            }

            return m.invoke(instance, args); // execute the original method
                                                // over the instance
        }
    };
    C proxy = (C) c.newInstance();

    ((Proxy) proxy).setHandler(mi);
    return (C) proxy;
 }
}

这是一个示例代码。让员工成为你的豆子：

public class Employee{
  private String name="John";
  private String surname="Smith";
  public String getName() {
    return name;
  }
  public void setName(String name) {
    this.name = name;
  }
  public String getSurname() {
    return surname;
  }
  public void setSurname(String surname) {
    this.surname = surname;
  }
};

这里的测试用例显示您可以为 POJO 创建代理，使用它的 getter，但不能使用它的 setter

@Test
public void testProxy() throws InstantiationException, IllegalAccessException{
    Employee aBean = new Employee();

    //I can modify the bean
    aBean.setName("Obi-Wan");
    aBean.setSurname("Kenobi");

    //create the protected java bean with the generic utility
    Employee protectedBean = Utils.createInmutableBean(Employee.class, aBean);

    //I can read
    System.out.println("Name: "+protectedBean.getName());
    System.out.println("Name: "+protectedBean.getSurname());

    //but I can't modify
    try{
        protectedBean.setName("Luke");
        protectedBean.setSurname("Skywalker");
        throw new RuntimeException("The test should not have reached this line!");
    }catch(Exception e){
        //I should be here
        System.out.println("The exception was expected! The bean should not be modified (exception message: "+e.getMessage()+")");
        assertEquals("Obi-Wan", protectedBean.getName());
        assertEquals("Kenobi", protectedBean.getSurname());
    }
}