Hessian和Java反序列化问题小结

Hessian反序列化问题

众所周知,Hessian框架提供的序列化方式,在性能上要优于Java自己的序列化方式。他将对象序列化,生成的字节数组的数量要相对于Java自带的序列化方式要更简洁。

目前公司的一个项目中,有RPC调用的需要,这里我们使用了公司自己的开源RPC框架Dubbo作为远程调用框架,进行业务方法的调用和对象的序列化。这里,我们没有对Dubbo做出特殊配置,Dubbo在Remoting层组件默认的序列化方式就是采用的Hessian协议处理。但是在真正部署测试时,走到需要远程调用的方式时,报出了一下异常(只截取了最核心的异常堆栈):

 

Caused by: com.alibaba.com.caucho.hessian.io.HessianProtocolException: 'com.alibaba.ais.bdc.person.vo.CountVO$CountObject' could not be instantiated
	at com.alibaba.com.caucho.hessian.io.JavaDeserializer.instantiate(JavaDeserializer.java:275)
	at com.alibaba.com.caucho.hessian.io.JavaDeserializer.readObject(JavaDeserializer.java:155)
	at com.alibaba.com.caucho.hessian.io.SerializerFactory.readObject(SerializerFactory.java:397)
	at com.alibaba.com.caucho.hessian.io.Hessian2Input.readObjectInstance(Hessian2Input.java:2070)
	at com.alibaba.com.caucho.hessian.io.Hessian2Input.readObject(Hessian2Input.java:2005)
	at com.alibaba.com.caucho.hessian.io.Hessian2Input.readObject(Hessian2Input.java:1990)
	at com.alibaba.com.caucho.hessian.io.CollectionDeserializer.readLengthList(CollectionDeserializer.java:93)
	at com.alibaba.com.caucho.hessian.io.Hessian2Input.readObject(Hessian2Input.java:1678)
	at com.alibaba.com.caucho.hessian.io.JavaDeserializer$ObjectFieldDeserializer.deserialize(JavaDeserializer.java:396)
	... 42 more
Caused by: java.lang.reflect.InvocationTargetException
	at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
	at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
	at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
	at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
	at com.alibaba.com.caucho.hessian.io.JavaDeserializer.instantiate(JavaDeserializer.java:271)
	... 50 more
Caused by: java.lang.IllegalArgumentException: [Assertion failed] - this argument is required; it must not be null
	at org.springframework.util.Assert.notNull(Assert.java:112)
	at org.springframework.util.Assert.notNull(Assert.java:123)
	at com.alibaba.ais.bdc.person.vo.CountVO$CountObject.(CountVO.java:101)
	... 55 more

从最下面的异常信息可以看出,CountObject这个内部类在对象初始化时,报了参数校验的失败。这个看一下CountObject的出问题的构造函数就一目了然了:

        public CountObject(SimplePerson simplePerson, String imagePrefix){
            Assert.notNull(simplePerson);
            if (StringUtils.isEmpty(imagePrefix)) {
                throw new IllegalArgumentException("imagePrefix [" + imagePrefix + "] is meaningless.");
            }
            this.id = simplePerson.getEmployeeId();
            this.name = simplePerson.getRealName();
            this.imagePath = StringUtils.isEmpty(simplePerson.getImagePathSuffix()) ? null : imagePrefix
                                                                                             + simplePerson.getImagePathSuffix();
        }

现在在构造函数的第一行的Assert就失败了。可是哪里调用这个构造函数导致失败呢?继续网上翻看异常堆栈给出的信息。可以看出在JavaDeserializer.instantiate中抛出了HessianProtocolException异常。进去看一下Hessian这块的源码如下:

  protected Object instantiate()
    throws Exception
  {
    try {
      if (_constructor != null)
	return _constructor.newInstance(_constructorArgs);
      else
	return _type.newInstance();
    } catch (Exception e) {
      throw new HessianProtocolException("'" + _type.getName() + "' could not be instantiated", e);
    }
  }

这里结合上面的异常堆栈可以知道,上面出问题的关键是_constructor_constructorArgs。这两个东东又到底是啥呢?继续来看代码:

  public JavaDeserializer(Class cl)
  {
    _type = cl;
    _fieldMap = getFieldMap(cl);

    _readResolve = getReadResolve(cl);

    if (_readResolve != null) {
      _readResolve.setAccessible(true);
    }

    Constructor []constructors = cl.getDeclaredConstructors();
    long bestCost = Long.MAX_VALUE;

    for (int i = 0; i < constructors.length; i++) {
      Class []param = constructors[i].getParameterTypes();
      long cost = 0;

      for (int j = 0; j < param.length; j++) {
	cost = 4 * cost;

	if (Object.class.equals(param[j]))
	  cost += 1;
	else if (String.class.equals(param[j]))
	  cost += 2;
	else if (int.class.equals(param[j]))
	  cost += 3;
	else if (long.class.equals(param[j]))
	  cost += 4;
	else if (param[j].isPrimitive())
	  cost += 5;
	else
	  cost += 6;
      }

      if (cost < 0 || cost > (1 << 48))
	cost = 1 << 48;

      cost += (long) param.length << 48;
      // _constructor will reference to the constructor with least parameters.
      if (cost < bestCost) {
        _constructor = constructors[i];
        bestCost = cost;
      }
    }

    if (_constructor != null) {
      _constructor.setAccessible(true);
      Class []params = _constructor.getParameterTypes();
      _constructorArgs = new Object[params.length];
      for (int i = 0; i < params.length; i++) {
        _constructorArgs[i] = getParamArg(params[i]);
      }
    }
  }

JavaDeserializer的构造方法中可以看出,这里_constructor会被赋予参数最少的那个构造器。再回过头去看看CountObject的构造器(就上面列出来的那一个),不难看出,这里的_constructor就是上面的那个构造器了。

  /**
   * Creates a map of the classes fields.
   */
  protected static Object getParamArg(Class cl)
  {
    if (! cl.isPrimitive())
      return null;
    else if (boolean.class.equals(cl))
      return Boolean.FALSE;
    else if (byte.class.equals(cl))
      return new Byte((byte) 0);
    else if (short.class.equals(cl))
      return new Short((short) 0);
    else if (char.class.equals(cl))
      return new Character((char) 0);
    else if (int.class.equals(cl))
      return Integer.valueOf(0);
    else if (long.class.equals(cl))
      return Long.valueOf(0);
    else if (float.class.equals(cl))
      return Float.valueOf(0);
    else if (double.class.equals(cl))
      return Double.valueOf(0);
    else
      throw new UnsupportedOperationException();
  }

参看上面的getParamArg方法,就可以知道,由于CountObject唯一的一个构造器的两个参数都不是基本类型,所以这里_constructorArgs所包含的值全部是null。

OK,到这里,上面的异常就搞清楚了,Hessian反序列化时,使用反射调用构造函数生成对象时,传入的参数不合法,造成了上面的异常。知道了原因,解决的方法也很简单,就是添加了一个无参的构造器给CountObject,于是上面的问题就解决了。。。

这里,需要注意的是,如果序列化机制使用的是Hessian,序列化的对象又没有提供默认的无参构造器时,需要注意上面类似的问题了。

Java本身反序列化问题

Java本身的反序列化机制虽然性能稍差一些,但本身使用的约束条件相对却要宽松一些,其实只要满足下面两条,一个类对象就是可以完美支持序列化机制了:


  1. 类实现java.io.Serializable接口。
  2. 类包含的所有属性都是实现了java.io.Serializable接口的,或者被标记为了transient。

对于构造函数本身没有任何约束。这里,Java序列化本身其实也是和new以及Java反射机制“平级”的实例化对象的方式。所以,对于单例模式的场景,还是需要考虑是否会有序列化因素造成的单例失效(因为他实例化对象不依赖于构造器,所以一个private的构造器显然没法阻止他的“胡作非为”)。当然,对于这种情况,也可以自己实现下面的方法:

 

private Object readResolve()

通过实现上面的方法,自己可以在其中明确指定,放回的对象的实例是哪一个。但对于通过如上方式保证的单例模式至少需要注意一下两点:


  1. readResolve方法的可见性(public/protected/private)问题:因为如果这个方法不是private的,就有可能被起子类直接继承过去。这可能造成你在反序列化子类对象时出错(因为这个方法返回了父类的某个固定的对象)。
  2. 使用readResolve方法时,往往比较容易返回某个固定的对象。但这其实和真正的对象反序列化其实是有点矛盾的。因为你反序列化对象时,多数场景都是希望恢复原来的对象的“状态”,而不是固定的某个对象。所以只要你的类内的属性有没有被标识成transient的,就要格外小心了。

鉴于上面所说的稍微复杂的现象,如果单纯的考虑单例的需要,更好的方式是通过枚举来实现,因为枚举至少可以在JVM层面,帮你保证每个枚举实例一定是单例的,即使使用反序列化机制,也无法绕过这个限制,所以可以帮你省不少心。

好了,上面扯的有点远了,关于Java本身的序列化机制,下面写了一个简单的把对象序列化成字节数组,再由字节数组反序列化回来的例子,看完之后应该会更明了一些:

 

public class Person implements Serializable {

    String name;
    int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    @Override
    public String toString() {
        return "Person{" +
            "name='" + name + '\'' +
            ", age=" + age +
            '}';
    }

    private static class Employee extends Person{

        String title;

        private Employee(String name, int age, String title) {
            super(name, age);
            this.title = title;
        }

        @Override
        public String toString() {
            return "Employee{" + "name='" + name + '\'' +
                ", age=" + age + '\'' +
                ", title='" + title + '\'' +
                '}';
        }
    }

    public static void main(String[] args) {
        byte[] bytes;
        Person person1 = new Person( "test1",20 );
        Person person2;
        Employee employee1 = new Employee( "employee1",25,"Manager" );
        Employee employee2;

        ByteArrayOutputStream byteOutputStream = null;
        ObjectOutputStream objectOutputStream = null;

        ByteArrayInputStream byteArrayInputStream = null;
        ObjectInputStream objectInputStream = null;

        try {
            //generate byteArray.
            byteOutputStream = new ByteArrayOutputStream( );
            objectOutputStream = new ObjectOutputStream( byteOutputStream);
            //serialize person1
            objectOutputStream.writeObject( person1 );
            //serialize employee1
            objectOutputStream.writeObject( employee1 );

            bytes = byteOutputStream.toByteArray();

            for (byte aByte : bytes) {
                System.out.print(aByte);
            }
            System.out.println();
            System.out.println("Bytes's length is :"+bytes.length);

            //generate Object from byteArray.
            byteArrayInputStream = new ByteArrayInputStream( bytes );
            objectInputStream = new ObjectInputStream( byteArrayInputStream );
            //deserialize person1
            person2 = (Person)objectInputStream.readObject();
            //deserialize employee1
            employee2 = (Employee)objectInputStream.readObject();
            System.out.println("person2 got from byteArray is : "+person2);
            System.out.println("employee2 got from byteArray is : "+employee2);

            System.out.println("person1's memory id :"+Integer.toHexString(person1.hashCode()));
            System.out.println("person2's memory id :"+Integer.toHexString(person2.hashCode()));
            System.out.println("employee1's memory id :"+Integer.toHexString(employee1.hashCode()));
            System.out.println("employee2's memory id :"+Integer.toHexString(employee2.hashCode()));

        } catch (IOException e) {
            e.printStackTrace();
        }catch ( ClassNotFoundException ce ){
            ce.printStackTrace();
        }
        finally {
            try {
                byteOutputStream.close();
                objectOutputStream.close();
                byteArrayInputStream.close();
                objectInputStream.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

上面代码执行的结果如下:

-84-19051151140329911110946115107121461191191194611510111410597108105122971161051111104680101114115111110-97-123-26-11-111120-40-115202730397103101760411097109101116018761069711897471089711010347831161141051101035912011200020116051161011151164911511404199111109461151071214611911911946115101114105971081051229711610511111046801011141151111103669109112108111121101101-11-66110-28-62-10611536201760511610511610810111301260112011301260000025116091011091121081111211011014911607779711097103101114
Bytes's length is :200
person2 got from byteArray is : Person{name='test1', age=20}
employee2 got from byteArray is : Employee{name='employee1', age=25', title='Manager'}
person1's memory id :29173ef
person2's memory id :96fa474
employee1's memory id :6c121f1d
employee2's memory id :95c083

最后再补充一个Java序列化规范的地址,有时间时再细读一下:http://docs.oracle.com/javase/7/docs/platform/serialization/spec/serial-arch.html  

     

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