上篇Hadoop之HDFS原理及文件上传下载源码分析（上）楼主主要介绍了hdfs原理及FileSystem的初始化源码解析， Client如何与NameNode建立RPC通信。本篇将继续介绍hdfs文件上传、下载源解析。
文件上传
先上文件上传的方法调用过程时序图：
其主要执行过程：
- FileSystem初始化，Client拿到NameNodeRpcServer代理对象，建立与NameNode的RPC通信（楼主上篇已经介绍过了）
- 调用FileSystem的create()方法，由于实现类为DistributedFileSystem,所有是调用该类中的create()方法
- DistributedFileSystem持有DFSClient的引用，继续调用DFSClient中的create()方法
- DFSOutputStream提供的静态newStreamForCreate()方法中调用NameNodeRpcServer服务端的create()方法并创建DFSOutputStream输出流对象返回
- 通过hadoop提供的IOUtil工具类将输出流输出到本地
下面我们来看下源码：
首先初始化文件系统，建立与服务端的RPC通信
HDFSDemo.java
OutputStream os = fs.create(new Path("/test.log"));
调用FileSystem的create()方法，由于FileSystem是一个抽象类，这里实际上是调用的该类的子类create()方法
//FileSystem.java
public abstract FSDataOutputStream create(Path f,
FsPermission permission,
boolean overwrite,
int bufferSize,
short replication,
long blockSize,
Progressable progress) throws IOException;
前面我们已经说过FileSystem.get()返回的是DistributedFileSystem对象，所以这里我们直接进入DistributedFileSystem：
//DistributedFileSystem.java
@Override
public FSDataOutputStream create(final Path f, final FsPermission permission,
final EnumSet<CreateFlag> cflags, final int bufferSize,
final short replication, final long blockSize, final Progressable progress,
final ChecksumOpt checksumOpt) throws IOException {
statistics.incrementWriteOps(1);
Path absF = fixRelativePart(f);
return new FileSystemLinkResolver<FSDataOutputStream>() {
@Override
public FSDataOutputStream doCall(final Path p)
throws IOException, UnresolvedLinkException {
final DFSOutputStream dfsos = dfs.create(getPathName(p), permission,
cflags, replication, blockSize, progress, bufferSize,
checksumOpt);
//dfs为DistributedFileSystem所持有的DFSClient对象，这里调用DFSClient中的create()方法
return dfs.createWrappedOutputStream(dfsos, statistics);
}
@Override
public FSDataOutputStream next(final FileSystem fs, final Path p)
throws IOException {
return fs.create(p, permission, cflags, bufferSize,
replication, blockSize, progress, checksumOpt);
}
}.resolve(this, absF);
}
DFSClient的create()返回一个DFSOutputStream对象：
//DFSClient.java
public DFSOutputStream create(String src,
FsPermission permission,
EnumSet<CreateFlag> flag,
boolean createParent,
short replication,
long blockSize,
Progressable progress,
int buffersize,
ChecksumOpt checksumOpt,
InetSocketAddress[] favoredNodes) throws IOException {
checkOpen();
if (permission == null) {
permission = FsPermission.getFileDefault();
}
FsPermission masked = permission.applyUMask(dfsClientConf.uMask);
if(LOG.isDebugEnabled()) {
LOG.debug(src + ": masked=" + masked);
}
//调用DFSOutputStream的静态方法newStreamForCreate，返回输出流
final DFSOutputStream result = DFSOutputStream.newStreamForCreate(this,
src, masked, flag, createParent, replication, blockSize, progress,
buffersize, dfsClientConf.createChecksum(checksumOpt),
getFavoredNodesStr(favoredNodes));
beginFileLease(result.getFileId(), result);
return result;
}
我们继续看下newStreamForCreate()中的业务逻辑：
//DFSOutputStream.java
static DFSOutputStream newStreamForCreate(DFSClient dfsClient, String src,
FsPermission masked, EnumSet<CreateFlag> flag, boolean createParent,
short replication, long blockSize, Progressable progress, int buffersize,
DataChecksum checksum, String[] favoredNodes) throws IOException {
TraceScope scope =
dfsClient.getPathTraceScope("newStreamForCreate", src);
try {
HdfsFileStatus stat = null;
boolean shouldRetry = true;
int retryCount = CREATE_RETRY_COUNT;
while (shouldRetry) {
shouldRetry = false;
try {
//这里通过dfsClient的NameNode代理对象调用NameNodeRpcServer中实现的create()方法
stat = dfsClient.namenode.create(src, masked, dfsClient.clientName,
new EnumSetWritable<CreateFlag>(flag), createParent, replication,
blockSize, SUPPORTED_CRYPTO_VERSIONS);
break;
} catch (RemoteException re) {
IOException e = re.unwrapRemoteException(
AccessControlException.class,
DSQuotaExceededException.class,
FileAlreadyExistsException.class,
FileNotFoundException.class,
ParentNotDirectoryException.class,
NSQuotaExceededException.class,
RetryStartFileException.class,
SafeModeException.class,
UnresolvedPathException.class,
SnapshotAccessControlException.class,
UnknownCryptoProtocolVersionException.class);
if (e instanceof RetryStartFileException) {
if (retryCount > 0) {
shouldRetry = true;
retryCount--;
} else {
throw new IOException("Too many retries because of encryption" +
" zone operations", e);
}
} else {
throw e;
}
}
}
Preconditions.checkNotNull(stat, "HdfsFileStatus should not be null!");
//new输出流对象
final DFSOutputStream out = new DFSOutputStream(dfsClient, src, stat,
flag, progress, checksum, favoredNodes);
out.start();//调用内部类DataStreamer的start()方法，DataStreamer继承Thread，所以说这是一个线程，从NameNode中申请新的block信息；
同时前面我们介绍hdfs原理的时候提到的流水线作业（Pipeline）也是在这里实现，有兴趣的同学可以去研究下，这里就不带大家看了
return out;
} finally {
scope.close();
}
}
到此，Client拿到了服务端的输出流对象，那么后面就容易了，都是一些简答的文件输出，输入流的操作（hadoop提供的IOUitl）。
文件下载
文件上传的大致流程与文件下载类似，与上传一样，我们先上程序方法调用时序图：
主要执行过程：
- FileSystem初始化，Client拿到NameNodeRpcServer代理对象，建立与NameNode的RPC通信（与前面一样）
- 调用FileSystem的open()方法，由于实现类为DistributedFileSystem,所有是调用该类中的open()方法
- DistributedFileSystem持有DFSClient的引用，继续调用DFSClient中的open()方法
- 实例化DFSInputStream输入流
- 调用openinfo()方法
- 调用fetchLocatedBlocksAndGetLastBlockLength()方法，抓取block信息并获取最后block长度
- 调用DFSClient中的getLocatedBlocks()方法，获取block信息
- 在callGetBlockLocations()方法中通过NameNode代理对象调用NameNodeRpcServer的getBlockLocations()方法
- 将block信息写入输出流
- 交给IOUtil,下载文件到本地
接下来，我们开始看源码：
首先任然是FileSystem的初始化，前面有，这里就不贴出来了，我们直接从DistributedFileSystem的open()开始看。
//DistributedFifeSystem.java
@Override
public FSDataInputStream open(Path f, final int bufferSize)
throws IOException {
statistics.incrementReadOps(1);
Path absF = fixRelativePart(f);
return new FileSystemLinkResolver<FSDataInputStream>() {
@Override
public FSDataInputStream doCall(final Path p)
throws IOException, UnresolvedLinkException {
final DFSInputStream dfsis =
dfs.open(getPathName(p), bufferSize, verifyChecksum);
//dfs为DFSClient对象，调用open()返回输入流
return dfs.createWrappedInputStream(dfsis);
}
@Override
public FSDataInputStream next(final FileSystem fs, final Path p)
throws IOException {
return fs.open(p, bufferSize);
}
}.resolve(this, absF);
}
DFSClient中并没有直接使用NameNode的代理对象，而是传给了DFSInputStream：
//DFSClient.java
public DFSInputStream open(String src, int buffersize, boolean verifyChecksum)
throws IOException, UnresolvedLinkException {
checkOpen();
TraceScope scope = getPathTraceScope("newDFSInputStream", src);
try {
//这里并没有直接通过NameNode的代理对象调用服务端的方法，直接new输入流并把当前对象作为参数传入
return new DFSInputStream(this, src, verifyChecksum);
} finally {
scope.close();
}
}
那么在DFSInputStream必须持有DFSClient的引用：
//DFSInputStream.java 构造
DFSInputStream(DFSClient dfsClient, String src, boolean verifyChecksum
) throws IOException, UnresolvedLinkException {
this.dfsClient = dfsClient;//只有DFSClient的引用
this.verifyChecksum = verifyChecksum;
this.src = src;
synchronized (infoLock) {
this.cachingStrategy = dfsClient.getDefaultReadCachingStrategy();
}
openInfo();//调openInfo()
}
openInfo()用来抓取block信息：
void openInfo() throws IOException, UnresolvedLinkException {
synchronized(infoLock) {
lastBlockBeingWrittenLength = fetchLocatedBlocksAndGetLastBlockLength();//抓取block信息
int retriesForLastBlockLength = dfsClient.getConf().retryTimesForGetLastBlockLength;//获取配置信息，尝试抓取的次数，楼主记得在2.6以前这里写的3；当然，现在的默认值也为3
while (retriesForLastBlockLength > 0) {
if (lastBlockBeingWrittenLength == -1) {
DFSClient.LOG.warn("Last block locations not available. "
+ "Datanodes might not have reported blocks completely."
+ " Will retry for " + retriesForLastBlockLength + " times");
waitFor(dfsClient.getConf().retryIntervalForGetLastBlockLength);
lastBlockBeingWrittenLength = fetchLocatedBlocksAndGetLastBlockLength();
} else {
break;
}
retriesForLastBlockLength--;
}
if (retriesForLastBlockLength == 0) {
throw new IOException("Could not obtain the last block locations.");
}
}
}
获取block信息：
//DFSInputStream.java
private long fetchLocatedBlocksAndGetLastBlockLength() throws IOException {
final LocatedBlocks newInfo = dfsClient.getLocatedBlocks(src, 0);
//回到DFSClient中来获取当前block信息
if (DFSClient.LOG.isDebugEnabled()) {
DFSClient.LOG.debug("newInfo = " + newInfo);
}
if (newInfo == null) {
throw new IOException("Cannot open filename " + src);
} if (locatedBlocks != null) {
Iterator<LocatedBlock> oldIter = locatedBlocks.getLocatedBlocks().iterator();
Iterator<LocatedBlock> newIter = newInfo.getLocatedBlocks().iterator();
while (oldIter.hasNext() && newIter.hasNext()) {
if (! oldIter.next().getBlock().equals(newIter.next().getBlock())) {
throw new IOException("Blocklist for " + src + " has changed!");
}
}
}
locatedBlocks = newInfo;
long lastBlockBeingWrittenLength = 0;
if (!locatedBlocks.isLastBlockComplete()) {
final LocatedBlock last = locatedBlocks.getLastLocatedBlock();
if (last != null) {
if (last.getLocations().length == 0) {
if (last.getBlockSize() == 0) {
return 0;
}
return -1;
}
final long len = readBlockLength(last);
last.getBlock().setNumBytes(len);
lastBlockBeingWrittenLength = len;
}
} fileEncryptionInfo = locatedBlocks.getFileEncryptionInfo();
//返回block开始写的位置
return lastBlockBeingWrittenLength;
}
回到DFSClient中：
DFSClient.java
@VisibleForTesting
public LocatedBlocks getLocatedBlocks(String src, long start, long length)
throws IOException {
TraceScope scope = getPathTraceScope("getBlockLocations", src);
try {
//这里NameNode作为参数传递到callGetBlockLocations()中
return callGetBlockLocations(namenode, src, start, length);
} finally {
scope.close();
}
}
调用服务端方法，返回block信息：
//DFSClient.java
static LocatedBlocks callGetBlockLocations(ClientProtocol namenode,
String src, long start, long length)
throws IOException {
try {
//看到这里，不用做过多的解释了吧？
return namenode.getBlockLocations(src, start, length);
} catch(RemoteException re) {
throw re.unwrapRemoteException(AccessControlException.class,
FileNotFoundException.class,
UnresolvedPathException.class);
}
}
最终将文件block相关信息写入输入流，通过工具类IOUtil输出到本地文件。
那关于hadoop之hdfs原理及文件上传下载源码解析就写到这里，下系列的文章，楼主会写一些关于mapreduce或者hive相关的文章分享给大家。
示例代码地址：https://github.com/LJunChina/hadoop