如何使用hadoop客户端

public class testCreate {
    public static void main(String[] args) throws IOException {
        System.setProperty("HADOOP_USER_NAME", "hdfs");
        String pathStr = "/home/hdp/shanshajia";
        Path path = new Path(pathStr);
        Configuration configuration = new HdfsConfiguration();
        FileSystem fs = path.getFileSystem(configuration);
    }
}

一般我们会这么使用，那么问题来了，FileSystem是个抽象类，我们到底用哪一个呢？或者说jvm怎么知道我们要加载哪些FileSystem的实现类呢？

Java SPI机制

SPI 全称 Service Provider Interface ，是 Java 提供的一套用来被第三方实现或者扩展的 API，它可以用来启用框架扩展和替换组件。

Java SPI 实际上是 基于接口的编程＋策略模式＋配置文件 组合实现的动态加载机制。

Java SPI 就是提供这样的一个机制：为某个接口寻找服务实现的机制。

将装配的控制权移到程序之外，在模块化设计中这个机制尤其重要。

所以 SPI 的核心思想就是解耦

Hadoop 客户端 SPI

当我们第一次视图创建一个FileSystem，会调用createFileSystem

  private static FileSystem createFileSystem(URI uri, Configuration conf
      ) throws IOException {
    Tracer tracer = FsTracer.get(conf);
    TraceScope scope = null;
    if (tracer != null) {
      scope = tracer.newScope("FileSystem#createFileSystem");
      scope.addKVAnnotation("scheme", uri.getScheme());
    }
    try {
      Class clazz = getFileSystemClass(uri.getScheme(), conf);
      if (clazz == null) {
        throw new IOException("No FileSystem for scheme: " + uri.getScheme());
      }
      FileSystem fs = (FileSystem)ReflectionUtils.newInstance(clazz, conf);
      fs.tracer = tracer;
      fs.initialize(uri, conf);
      return fs;
    } finally {
      if (scope != null) scope.close();
    }
  }

  public static Class getFileSystemClass(String scheme,
      Configuration conf) throws IOException {
    if (!FILE_SYSTEMS_LOADED) {
      loadFileSystems();
    }
    Class clazz = null;
    if (conf != null) {
      clazz = (Class) conf.getClass("fs." + scheme + ".impl", null);
    }
    if (clazz == null) {
      clazz = SERVICE_FILE_SYSTEMS.get(scheme);
    }
    if (clazz == null) {
      throw new IOException("No FileSystem for scheme: " + scheme);
    }
    return clazz;
  }

1、尝试使用配置的fs

    fs.hdfs.impl
    DistributedFileSystem

2、如果配置里没有，这个时候用加载的

hadoop-hdfs-project/hadoop-hdfs/src/main/resources/META-INF/services，可以看到，这个目录下有一个org.apache.hadoop.fs.FileSystem文件

# Licensed to the Apache Software Foundation (ASF) under one or more
# contributor license agreements.  See the NOTICE file distributed with
# this work for additional information regarding copyright ownership.
# The ASF licenses this file to You under the Apache License, Version 2.0
# (the "License"); you may not use this file except in compliance with
# the License.  You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
org.apache.hadoop.hdfs.DistributedFileSystem
org.apache.hadoop.hdfs.web.HftpFileSystem
org.apache.hadoop.hdfs.web.HsftpFileSystem
org.apache.hadoop.hdfs.web.WebHdfsFileSystem
org.apache.hadoop.hdfs.web.SWebHdfsFileSystem

 通过 loadFileSystems这个方法会加载文件中的所有类，并且根据scheme生成一个map，key是scheme，value是fs

  private static void loadFileSystems() {
    synchronized (FileSystem.class) {
      if (!FILE_SYSTEMS_LOADED) {
        ServiceLoader serviceLoader = ServiceLoader.load(FileSystem.class);
        Iterator it = serviceLoader.iterator();
        while (it.hasNext()) {
          FileSystem fs = null;
          try {
            fs = it.next();
            try {
              SERVICE_FILE_SYSTEMS.put(fs.getScheme(), fs.getClass());
            } catch (Exception e) {
              LOG.warn("Cannot load: " + fs + " from " +
                  ClassUtil.findContainingJar(fs.getClass()), e);
            }
          } catch (ServiceConfigurationError ee) {
            LOG.warn("Cannot load filesystem", ee);
          }
        }
        FILE_SYSTEMS_LOADED = true;
      }
    }
  }