看下hdfs的读写原理，主要是打开FileSystem,获得InputStream or OutputStream；
那么主要用到的FileSystem类是一个实现了文件系统的抽象类，继承来自org.apache.hadoop.conf.Configured，并且实现了Close able接口，可以适用于如本地文件系统file://，ftp,hdfs等多种文件系统，所以呢
若是自己要实现一个系统可以通过继承这个类，做出相应的配置，并且实现相应的抽象方法；
public abstract class FileSystem extends Configured implements Closeable {
public static FileSystem get(Configuration conf) throws IOException {
return get(getDefaultUri(conf), conf);
}
public static URI getDefaultUri(Configuration conf) {
return URI.create(fixName(conf.get(FS_DEFAULT_NAME_KEY, DEFAULT_FS)));
}
public static FileSystem get(URI uri, Configuration conf) throws IOException {
String scheme = uri.getScheme();
String authority = uri.getAuthority();
if (scheme == null && authority == null) { // use default FS
return get(conf);
}
if (scheme != null && authority == null) { // no authority
URI defaultUri = getDefaultUri(conf);
if (scheme.equals(defaultUri.getScheme()) // if scheme matches default
&& defaultUri.getAuthority() != null) { // & default has authority
return get(defaultUri, conf); // return default
}
}
String disableCacheName www.xycheng178.com= String.format("fs.%s.impl.disable.cache", scheme);
if (conf.getBoolean(disableCacheName, false)) {
return createFileSystem(uri, conf);
}
return CACHE.get(uri, conf);
}
}
从部分源码看下，get方法根据conf获取具体的文件系统对象，，而get(uri,conf)方法基于uri和conf创建文件系统对象；
那么看一个简单的应用，用java的api打开一个文件，并且打印出来
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.FSDataInputStream;
import org.apache.hadoop.fs.Path;
public class OpenMethod {
public static void main(String args[]) throws Exception{
Configuration conf = new Configuration();
conf.set("fs.defaultFs", "hdfs://10.192.4.33:9000/");//配置conf
FileSystem fs = FileSystem.get(conf);//根据创建FileSystem对象
Path path = new Path("/home/overshow/hehe.txt");
FSDataInputStream fis = fs.open(path);
byte b[] = new byte[200];
int i = fis.read(b);
System.out.println(new String(b,0,i));
}
}
这里还有三个需要注意的类，一个是Path，一个是FSDataInputStream，一个是conf
let me show you something
first: Configuration
public class Configuration implements Iterable<Map.Entry<String,String>>,Writable {
public void set(String name, String value) {
set(name, value, www.ysyl157.com null);
}
/**
* Set the <code>value</code> of the <www.mcyllpt.com code>www.dasheng178.com name</code> property. If
* <code>name</code> is deprecated, it also sets the <code>value</code> to
* the keys that replace the deprecated key. Name will be trimmed before put
* into configuration.
*
* @param name property name.
* @param value property value.
* @param source the place that this configuration value came from
* (For debugging).
* @throws IllegalArgumentException when the value or name is null.
*/
}
这个类是作业的配置信息类，通过Configuration可以实现在多个mapper和多个reducer任务之间共享信息，所以任何作用的配置信息必须通过Configuration传递，该类实现了Iterable和Writable两个接口，首先Iterable是迭代出Configuration对象加载到内存中的所有name-value键值对。而Writable是为了实现hadoop框架要求的序列化，可以将内存中的name-value序列化到硬盘；其中的set方法设置Configuration的名称和链接；
而Path类继承了fs类，
public class Path implements www.michenggw.com/ Comparable {
private void checkPathArg( String path ) throws IllegalArgumentException {
// disallow construction of a Path from an empty string
if ( path == null ) {
throw new IllegalArgumentException(
"Can not create a Path from a null string");
}
if( path.length() =www.leyouzaixian2.com= 0 ) {
throw new IllegalArgumentException(
"Can not create a Path from an empty string");
}
}
/** Construct a path from a String. Path strings are URIs, but with
* unescaped elements and some additional normalization. */
public Path(String pathString) throws IllegalArgumentException {
checkPathArg( pathString );
// We can‘t use ‘new URI(String)‘ directly, since it assumes things are
// escaped, which we don‘t require of Paths.
// add a slash in front of paths with Windows drive letters
if (hasWindowsDrive(pathString) && pathString.charAt(0) != ‘/‘) {
pathString = "/" + pathString;
}
// parse uri components
String scheme = null;
String authority = null;
int start = 0;
// parse uri scheme, if any
int colon = pathString.indexOf(‘:‘);
int slash = pathString.indexOf(‘/‘);
if ((colon != -1) &&
((slash == -1) || (colon < slash))) { // has a scheme
scheme = pathString.substring(0, colon);
start = colon+1;
}
// parse uri authority, if any
if (pathString.startsWith("//", start) &&
(pathString.length()-start > 2)) { // has authority
int nextSlash = pathString.indexOf(‘/‘, start+2);
int authEnd = nextSlash > 0 ? nextSlash : pathString.length();
authority = pathString.substring(start+2, authEnd);
start = authEnd;
}
// uri path is the rest of the string -- query & fragment not supported
String path = pathString.substring(start, pathString.length());
initialize(scheme, authority, path, null);
}
/**
* Construct a path from a URI
*/
public Path(URI aUri) {
uri = aUri.normalize(www.hengy178.com);
}
}
好吧，其实就是设置了hdfs的地址；
最后一个类，FSDataInputStream，额，不想看，太长了，
用fs的open方法创建一个FSDataInputStream类的实例，然后简单来说，读文件的流程就是，客户端到最近的（Namenode说了算）DATa Node上调用FSDataInputStream的read方法，通过反复的调用read方法，将数据从DataNode传递到客户端。
值得一提的是，它创建string的那个构造方法，我找了半天源码，似乎是这个，
public String(byte bytes[www.mengzhidu178.com], int offset, int length) {
checkBounds(bytes, offset,www.gangchengyuLe178.com length);
this.value = StringCoding.decode(bytes, offset, length);
}
/**
* Constructs a new {@code String} by decoding the specified array of bytes
* using the platform‘s www.yongxinzaixian.cn default charset. The length of the new {@code
* String} is a function of the charset, and hence may not be equal to the
* length of the byte array.
*
* <p> The behavior of this constructor when the given bytes are not valid
* in the default charset is unspecified. The {@link
* java.nio.charset.CharsetDecoder} class should be used when more control
* over the decoding process is required.
*
* @param bytes
* The bytes to be decoded into characters
*
* @since JDK1.1
**/
#######################################################################
写文件流程差不多一致，不过用到的是另外一个输出流的类FSDataOutputStream;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.FSDataOutputStream;
import org.apache.hadoop.fs.Path;
public class Create_Method {
public static void main(String args[]) throws Exception{
Configuration conf = new Configuration();
conf.set("fs.defaultFS", "hdfs://10.192.4.33:9000/");
FileSystem fs = FileSystem.get(conf);
FSDataOutputStream fos = fs.create(new Path("/words.txt"));
fos.writeChars("hello world");
}
}
当然，fs还有一个用处就是查看文件目录，但是注意它的类型，是一个特殊的可迭代对象；
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.LocatedFileStatus;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.RemoteIterator;
public class listStatus {
public static void main(String args[])throws Exception{
Configuration conf = new Configuration();
conf.set("fs.defaultFS", "hdfs://10.192.4.33:9000/data");
FileSystem fs = FileSystem.get(conf);
Path path = new Path("/");
RemoteIterator<LocatedFileStatus> list = fs.listFiles(path, true);
while(list.hasNext()) {
System.out.println(list.next());
}
}
}
看下listFiles方法的源码
public RemoteIterator<LocatedFileStatus> listFiles(
final Path f, final boolean recursive)
throws FileNotFoundException, IOException {
return new RemoteIterator<LocatedFileStatus>() {
private Stack<RemoteIterator<LocatedFileStatus>> itors =
new Stack<RemoteIterator<LocatedFileStatus>>();
private RemoteIterator<LocatedFileStatus> curItor =
listLocatedStatus(f);
private LocatedFileStatus curFile;
@Override
public boolean hasNext() throws IOException {
while (curFile == null) {
if (curItor.hasNext()) {
handleFileStat(curItor.next());
} else if (!itors.empty()) {
curItor = itors.pop();
} else {
return false;
}
}
return true;
}
/**
* Process the input stat.
* If it is a file, return the file stat.
* If it is a directory, traverse the directory if recursive is true;
* ignore it if recursive is false.
* @param stat input status
* @throws IOException if any IO error occurs
*/
原文地址：https://www.cnblogs.com/qwangxiao/p/9951451.html