這幾天在學習Hadoop，下載的是2.9.0的版本。期間，遇到了很多問題，其中有一個問題是在Intellij IDEA中配置Maven依賴項時遇到的。在網上的教程裡，配置方式如下：

<dependencies>
    <dependency>
        <groupId>org.apache.hadoop</groupId>
        <artifactId>hadoop-core</artifactId>
        <version>1.2.1</version>
    </dependency
 
>
    <dependency>
        <groupId>org.apache.hadoop</groupId>
        <artifactId>hadoop-common</artifactId>
        <version>2.7.2</version>
    </dependency>
</dependencies>

但是這樣的配置在執行時報瞭如下錯誤：

Exception in thread "main" org.apache.hadoop.ipc
 
.RemoteException: Server IPC version 9 cannot communicate with client version 4
    at org.apache.hadoop.ipc.Client.call(Client.java:1113)
    at org.apache.hadoop.ipc.RPC$Invoker.invoke(RPC.java:229)
    at com.sun.proxy.$Proxy1.getProtocolVersion(Unknown Source)
    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)
    at com.sun.proxy.$Proxy1.getProtocolVersion(Unknown Source)
    at org.apache.hadoop.ipc.RPC.checkVersion(RPC.java:422)
    at org.apache.hadoop.hdfs.DFSClient.createNamenode(DFSClient.java:183)
    at org.apache.hadoop.hdfs.DFSClient.<init>(DFSClient.java:281)
    at org.apache.hadoop.hdfs.DFSClient.<init>(DFSClient.java:245)
    at org.apache.hadoop.hdfs.DistributedFileSystem.initialize(DistributedFileSystem.java:100)
    at org.apache.hadoop.fs.FileSystem.createFileSystem(FileSystem.java:1446)
    at org.apache.hadoop.fs.FileSystem.access$200(FileSystem.java:67)
    at org.apache.hadoop.fs.FileSystem$Cache.get(FileSystem.java:1464)
    at org.apache.hadoop.fs.FileSystem.get(FileSystem.java:263)
    at org.apache.hadoop.fs.Path.getFileSystem(Path.java:187)
    at Main.main(Main.java:20)

這是因為，我在配置中使用的hadoop-core項還是1.2.1版本的，早已不支援Hadoop 2.9.0。而且由於Hadoop配置方式變更，上述問題不能通過改變hadoop-core版本解決。正確方式是使用hadoop-client和hadoop-hdfs兩個依賴項取代hadoop-core。配置方式如下:

<dependencies>
    <dependency>
        <groupId>org.apache.hadoop</groupId>
        <artifactId>hadoop-hdfs</artifactId>
        <version>2.9.0</version>
    </dependency>

    <dependency>
        <groupId>org.apache.hadoop</groupId>
        <artifactId>hadoop-client</artifactId>
        <version>2.9.0</version>
    </dependency>

    <dependency>
        <groupId>org.apache.hadoop</groupId>
        <artifactId>hadoop-common</artifactId>
        <version>2.9.0</version>
    </dependency>
</dependencies>

這下子就不會出以上問題了。