Dubbo Hessian2序列化问题

问题

定义了一个接口，返回值为Map<String, Byte>，consumer收到结果后，使用Byte value = map.get(key)获取结果，抛出了如下类型转换异常：
如下:

java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.Byte    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)    at java.lang.reflect.Method.invoke(Method.java:498)

当时很奇怪，为什么返回值已经是Map<String, Byte>了，还会抛类型转换异常呢。

分析

首先想到的是通过debug确认返回的类型是否是Byte，设置断点后发现返回的Map的value的类型并不是Byte而是Integer，那么为什么会是Integer呢？
这里猜测这里很可能是dubbo序列化导致的，dubbo默认使用Hessaian2作为序列化协议，所以去看了下Hessian2的序列化实现，果不其然，byte会被反序列化成Integer

代码在Hessian2Input的readObject方法：

/**   * Reads an arbitrary object from the input stream when the type   * is unknown.   */  public Object readObject()    throws IOException  {    int tag = _offset < _length ? (_buffer[_offset++] & 0xff) : read();    switch (tag) {    case 'N':      return null;    case 'T':      return Boolean.valueOf(true);    case 'F':      return Boolean.valueOf(false);      // direct integer    case 0x80: case 0x81: case 0x82: case 0x83:    case 0x84: case 0x85: case 0x86: case 0x87:    case 0x88: case 0x89: case 0x8a: case 0x8b:    case 0x8c: case 0x8d: case 0x8e: case 0x8f:    case 0x90: case 0x91: case 0x92: case 0x93:    case 0x94: case 0x95: case 0x96: case 0x97:    case 0x98: case 0x99: case 0x9a: case 0x9b:    case 0x9c: case 0x9d: case 0x9e: case 0x9f:    case 0xa0: case 0xa1: case 0xa2: case 0xa3:    case 0xa4: case 0xa5: case 0xa6: case 0xa7:    case 0xa8: case 0xa9: case 0xaa: case 0xab:    case 0xac: case 0xad: case 0xae: case 0xaf:    case 0xb0: case 0xb1: case 0xb2: case 0xb3:    case 0xb4: case 0xb5: case 0xb6: case 0xb7:    case 0xb8: case 0xb9: case 0xba: case 0xbb:    case 0xbc: case 0xbd: case 0xbe: case 0xbf:      return Integer.valueOf(tag - BC_INT_ZERO);      /* byte int */    case 0xc0: case 0xc1: case 0xc2: case 0xc3:    case 0xc4: case 0xc5: case 0xc6: case 0xc7:    case 0xc8: case 0xc9: case 0xca: case 0xcb:    case 0xcc: case 0xcd: case 0xce: case 0xcf:      return Integer.valueOf(((tag - BC_INT_BYTE_ZERO) << 8) + read()); //byte转换成了Integer      //省略其他代码

为了更好说明Hessian2序列化和反序列的问题，可以使用其序列化和反序列化
测试代码如下：

import com.alibaba.com.caucho.hessian.io.Hessian2Input;import com.alibaba.com.caucho.hessian.io.Hessian2Output;import com.google.common.collect.Maps;import org.junit.Test;import java.io.ByteArrayInputStream;import java.io.ByteArrayOutputStream;import java.io.IOException;import java.util.Map;public class Hessian2SerialzeTest {    @Test    public void test() throws IOException {        Map<String, Byte> map = Maps.newHashMap();        map.put("weather", (byte) 1);        byte[] bytes = serialize(map);        Map<String, Byte> object = (Map<String, Byte>) deserialize(bytes);        //这里会抛异常        Byte weather = object.get("weather");        System.out.println(weather);    }    public static byte[] serialize(Object obj) throws IOException {        ByteArrayOutputStream os = new ByteArrayOutputStream();        Hessian2Output ho = new Hessian2Output(os);        byte[] cc = null;        try {            if(obj==null) throw new NullPointerException();            ho.writeObject(obj);            ho.flushBuffer();            cc=os.toByteArray();        } catch (Exception e) {            e.printStackTrace();        }finally{            ho.close();        }        return cc;    }    public static Object deserialize(byte[] by) throws IOException{        try {            if(by==null) throw new NullPointerException();            ByteArrayInputStream is = new ByteArrayInputStream(by);            Hessian2Input hi = new Hessian2Input(is);            return hi.readObject();        } catch (Exception e) {            e.printStackTrace();        }        return null;    }}

运行该test，再一次抛出了Integer转Byte失败的异常，这就证明了我们的猜测，Hessian2序列化确实存在这个问题。
那么问题清晰了，解决就很自然了，要么换一种序列化协议，要么用Object接受Byte，然后把object强转成Byte（证实可行）。