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<? extends FileSystem> getFileSystemClass(String scheme,Configuration conf) throws IOException {if (!FILE_SYSTEMS_LOADED) {loadFileSystems();}Class<? extends FileSystem> clazz = null;if (conf != null) {clazz = (Class<? extends FileSystem>) 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
<property><name>fs.hdfs.impl</name><value>DistributedFileSystem</value>
</property>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
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# 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<FileSystem> serviceLoader = ServiceLoader.load(FileSystem.class);Iterator<FileSystem> 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;}}}
