Hessian

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简介

Hessian是一个由Caucho公司开发的轻量级二进制RPC协议，使用简单的方法提供了RMI的功能。相比WebService，Hessian更简单、快捷。Hessian采用的是二进制RPC协议，所以它很适合于发送二进制数据。

Hessian可通过Servlet提供远程服务，需要将url请求映射到Hessian服务，也可与SpringMVC框架整合，通过它的 DispatcherServlet可将匹配的url请求转发到Hessian服务。Hessian的RPC功能完全使用动态代理来实现的，建议采用面向接口编程，通过接口暴露Hessian服务。类似的框架有DUBBO、SOFA。

常见问题

  1. 控制台警告
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    警告: Hessian/Burlap: 'java.lang.Enum' is an unknown class...
    java.lang.RuntimeException: Class java.lang.Enum is not an enum.

解决办法：把两个项目与Hessian相关类的包名保持一致

  1. Hessian上传

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    public interface UploadService {

        // 注意：使用Hessian时需要把inputStream参数放在最后
        void upload(String path, InputStream inputStream);

    }

  2. Hessian下载

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    public interface DownloadService {

        InputStream download(String filename);

    }

下载时如果遇到以下问题，则可能是由Hessian的一个Bug导致的

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java.io.IOException: stream is closed
	at sun.net.www.http.ChunkedInputStream.ensureOpen(ChunkedInputStream.java:174)
	at sun.net.www.http.ChunkedInputStream.read(ChunkedInputStream.java:673)
	at java.io.FilterInputStream.read(FilterInputStream.java:133)
	at sun.net.www.protocol.http.HttpURLConnection$HttpInputStream.read(HttpURLConnection.java:3066)
	at com.caucho.hessian.io.Hessian2Input.readBuffer(Hessian2Input.java:2783)
	at com.caucho.hessian.io.Hessian2Input.read(Hessian2Input.java:2729)
	at com.caucho.hessian.io.Hessian2Input$ReadInputStream.read(Hessian2Input.java:2917)
	at java.io.InputStream.read(InputStream.java:101)
  ...

解决办法：覆盖Hessian包中的HessianProxy类(不建议直接修改源代码后重新打包)，添加如下代码：

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Object value = in.readReply(method.getReturnType());
// 以下是需要添加的Patch Code
if (value instanceof InputStream) {
    value = new ResultInputStream(conn, is, in, (InputStream) value);
    is = null;
    conn = null;
}

此外，还需要在项目的pom.xml中添加如下配置：

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<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-dependency-plugin</artifactId>
            <executions>
                <execution>
                    <id>unpack</id>
                    <phase>generate-sources</phase>
                    <goals>
                        <goal>unpack</goal>
                    </goals>
                    <configuration>
                        <artifactItems>
                            <artifactItem>
                                <groupId>com.caucho</groupId>
                                <artifactId>hessian</artifactId>
                                <version>${hessian.version}</version>
                                <type>jar</type>
                                <overWrite>true</overWrite>
                                <outputDirectory>${project.build.directory}/classes</outputDirectory>
                                <excludes>**/HessianProxy.class</excludes>
                            </artifactItem>
                        </artifactItems>
                    </configuration>
                </execution>
            </executions>
        </plugin>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-war-plugin</artifactId>
            <configuration>
                <packagingExcludes>WEB-INF/lib/hessian-${hessian.version}.jar</packagingExcludes>
            </configuration>
        </plugin>
    </plugins>
</build>