scan的调用代码示例如下：
// 创建HBase配置config
Configuration config = HBaseConfiguration.create();
config.set("hbase.zookeeper.quorum", "192.168.1.226");// zookeeper部署的服务器IP
config.set("hbase.zookeeper.property.clientPort", "2181");// zookeeper允许连接的客户端端口号
// 定义HBase连接
Connection connection = null;
try {
// 获取HBase数据库连接
connection = ConnectionFactory.createConnection(config);
// 输出连接建立结果
while (!connection.isClosed()) {
break;
}
// 获取HBase数据库表
Table table = connection.getTable(TableName.valueOf("test_table"));
// 构造Scan实例
Scan scan = new Scan();
scan.addColumn(Bytes.toBytes("family1"), Bytes.toBytes("cloumn1"));
// 获取查询结果
ResultScanner result = table.getScanner(scan);
// 解析查询结果
for (Result r : result) {
// 此处为处理Result的代码
byte[] row = r.getRow();
if(row.length == 0){
//...
}
}
result.close();
table.close();
table = null;
scan = null;
} catch (IOException e) {
e.printStackTrace();
} finally {
if (connection != null) {
try {
connection.close();
} catch (IOException e) {
e.printStackTrace();
}
}
} 下面，我们对scan的整个流程进行分析。
首先从Table的getScanner(Scan scan)方法入手，它的定义如下：
/**
* Returns a scanner on the current table as specified by the {@link Scan}
* object.
* Note that the passed {@link Scan}'s start row and caching properties
* maybe changed.
*
* @param scan A configured {@link Scan} object.
* @return A scanner.
* @throws IOException if a remote or network exception occurs.
* @since 0.20.0
*/
ResultScanner getScanner(Scan scan) throws IOException; 它的实现是由HTable来完成的，源码如下：
/**
* The underlying {@link HTable} must not be closed.
* {@link HTableInterface#getScanner(Scan)} has other usage details.
*
* HBase中scan的入口方法
*/
@Override
public ResultScanner getScanner(final Scan scan) throws IOException {
// small scan不可以设置batch
if (scan.getBatch() > 0 && scan.isSmall()) {
throw new IllegalArgumentException("Small scan should not be used with batching");
}
// 设置caching
// 取参数“hbase.client.scanner.caching”，如果参数未配置，则默认为100
if (scan.getCaching() <= 0) {
scan.setCaching(getScannerCaching());
}
// 取参数“HBASE_CLIENT_SCANNER_MAX_RESULT_SIZE_KEY”，如果参数未配置，则默认为2 * 1024 * 1024
if (scan.getMaxResultSize() <= 0) {
scan.setMaxResultSize(scannerMaxResultSize);
}
/**
* scan总共分为四种类型：
* 1、reversed、small--ClientSmallReversedScanner
* 2、reversed、big--ReversedClientScanner
* 3、notReversed、small--ClientSmallScanner
* 4、notReversed、big--ClientScanner
*/
if (scan.isReversed()) {// 反向扫描
if (scan.isSmall()) {
return new ClientSmallReversedScanner(getConfiguration(), scan, getName(),
this.connection, this.rpcCallerFactory, this.rpcControllerFactory,
pool, tableConfiguration.getReplicaCallTimeoutMicroSecondScan());
} else {
return new ReversedClientScanner(getConfiguration(), scan, getName(),
this.connection, this.rpcCallerFactory, this.rpcControllerFactory,
pool, tableConfiguration.getReplicaCallTimeoutMicroSecondScan());
}
}
if (scan.isSmall()) {
return new ClientSmallScanner(getConfiguration(), scan, getName(),
this.connection, this.rpcCallerFactory, this.rpcControllerFactory,
pool, tableConfiguration.getReplicaCallTimeoutMicroSecondScan());
} else {
return new ClientScanner(getConfiguration(), scan, getName(), this.connection,
this.rpcCallerFactory, this.rpcControllerFactory,
pool, tableConfiguration.getReplicaCallTimeoutMicroSecondScan());
}
} 这里，我们先只研究ClientScanner，其他三种以后再说。
/**
* Create a new ClientScanner for the specified table Note that the passed {@link Scan}'s start
* row maybe changed changed.
* @param conf The {@link Configuration} to use.
* @param scan {@link Scan} to use in this scanner
* @param tableName The table that we wish to scan
* @param connection Connection identifying the cluster
* @throws IOException
*/
public ClientScanner(final Configuration conf, final Scan scan, final TableName tableName,
ClusterConnection connection, RpcRetryingCallerFactory rpcFactory,
RpcControllerFactory controllerFactory, ExecutorService pool, int primaryOperationTimeout) throws IOException {
if (LOG.isTraceEnabled()) {
LOG.trace("Scan table=" + tableName
+ ", startRow=" + Bytes.toStringBinary(scan.getStartRow()));
}
// 设置scan、tableName等成员变量
this.scan = scan;
this.tableName = tableName;
this.lastNext = System.currentTimeMillis();
this.connection = connection;
this.pool = pool;
this.primaryOperationTimeout = primaryOperationTimeout;
// 重试次数，取参数“hbase.client.retries.number”，如果参数未配置，则默认为31
this.retries = conf.getInt(HConstants.HBASE_CLIENT_RETRIES_NUMBER,
HConstants.DEFAULT_HBASE_CLIENT_RETRIES_NUMBER);
if (scan.getMaxResultSize() > 0) {
this.maxScannerResultSize = scan.getMaxResultSize();
} else {
this.maxScannerResultSize = conf.getLong(
HConstants.HBASE_CLIENT_SCANNER_MAX_RESULT_SIZE_KEY,
HConstants.DEFAULT_HBASE_CLIENT_SCANNER_MAX_RESULT_SIZE);
}
// scanner超时时间
this.scannerTimeout = HBaseConfiguration.getInt(conf,
HConstants.HBASE_CLIENT_SCANNER_TIMEOUT_PERIOD,
HConstants.HBASE_REGIONSERVER_LEASE_PERIOD_KEY,
HConstants.DEFAULT_HBASE_CLIENT_SCANNER_TIMEOUT_PERIOD);
// check if application wants to collect scan metrics
initScanMetrics(scan);
// Use the caching from the Scan. If not set, use the default cache setting for this table.
// 处理caching
if (this.scan.getCaching() > 0) {
this.caching = this.scan.getCaching();
} else {
this.caching = conf.getInt(
HConstants.HBASE_CLIENT_SCANNER_CACHING,
HConstants.DEFAULT_HBASE_CLIENT_SCANNER_CACHING);
}
// 初始化caller
// caller为RpcRetryingCaller类型
this.caller = rpcFactory.<Result[]> newCaller();
// rpcControllerFactory为RpcControllerFactory类型
this.rpcControllerFactory = controllerFactory;
this.conf = conf;
// 初始化scanner
initializeScannerInConstruction();
} ClientScanner的构造方法中，首先是对各种成员变量赋值，比如scan、tableName、connection等，然后是处理maxScannerResultSize、scannerTimeout、caching等scan需要用到的各种参数，这些都没有什么好说的。
接下来，是初始化两个重要的变量caller和rpcControllerFactory，caller为RpcRetryingCaller类型的，rpcControllerFactory为RpcControllerFactory类型的。
最后调用initializeScannerInConstruction()方法，ok，我们也跟着继续。
protected void initializeScannerInConstruction() throws IOException{
// initialize the scanner
// 初始化scanner
nextScanner(this.caching, false);
} 紧接着，调用nextScanner()方法，注意，传入两个参数，一个是ClientScanner对象生成时的caching，另外一个是false。
这个caching，如果在构造Scan对象时没有设置，则取参数hbase.client.scanner.caching配置的值，参数未配置则默认为100，它的含义是每次RPC请求的最大行数。
继续追踪nextScanner()方法，完整的代码如下：
/*
* Gets a scanner for the next region. If this.currentRegion != null, then
* we will move to the endrow of this.currentRegion. Else we will get
* scanner at the scan.getStartRow(). We will go no further, just tidy
* up outstanding scanners, if <code>currentRegion != null</code> and
* <code>done</code> is true.
* @param nbRows
* @param done Server-side says we're done scanning.
*/
protected boolean nextScanner(int nbRows, final boolean done)
throws IOException {
// Close the previous scanner if it's open
// 关闭之前打开的scanner
// 第一次调用时，callable因为没有初始化，所以肯定是空的，为null，此处会跳过
// 什么时候callable被赋值，而什么时候callable又被清空呢？
// 关闭上一个callable
if (this.callable != null) {
// 调用setClose()方法将callable中的currentScannerCallable的closed设置为true
// 将ScannerCallableWithReplicas类型的callable中ScannerCallable类型的成员变量中的closed设置为true
this.callable.setClose();
// 调用call()方法，发起一次请求，此时callable不为空，且其currentScannerCallable中closed为true
// 最终调用其currentScannerCallable的call()方法，因为closed为true，只执行其close()方法
call(scan, callable, caller, scannerTimeout);
// 将callable设置为null，下次就不会执行该模块了
this.callable = null;
}
// Where to start the next scanner
// 从哪里开始下一个scanner
byte [] localStartKey;
// if we're at end of table, close and return false to stop iterating
// 如果我们处于表的末尾，关闭并返回false，以停止此次迭代
if (this.currentRegion != null) {
byte [] endKey = this.currentRegion.getEndKey();
// done，或者endKey为空，或者endKey>=stopRow
if (endKey == null ||
Bytes.equals(endKey, HConstants.EMPTY_BYTE_ARRAY) ||
checkScanStopRow(endKey) ||
done) {
// 关闭
close();
if (LOG.isTraceEnabled()) {
LOG.trace("Finished " + this.currentRegion);
}
// 返回false
return false;
}
// localStartKey设置为当前Region的endKey
localStartKey = endKey;
if (LOG.isTraceEnabled()) {
LOG.trace("Finished " + this.currentRegion);
}
} else {
localStartKey = this.scan.getStartRow();
}
if (LOG.isDebugEnabled() && this.currentRegion != null) {
// Only worth logging if NOT first region in scan.
LOG.debug("Advancing internal scanner to startKey at '" +
Bytes.toStringBinary(localStartKey) + "'");
}
try {
// callable为ScannerCallableWithReplicas类型的
// 获取一个新的callable
callable = getScannerCallable(localStartKey, nbRows);
// Open a scanner on the region server starting at the
// beginning of the region
// 在localStartKey所在Region的RegionServer上打开一个scanner
call(scan, callable, caller, scannerTimeout);
// 设置currentRegion
this.currentRegion = callable.getHRegionInfo();
if (this.scanMetrics != null) {
this.scanMetrics.countOfRegions.incrementAndGet();
}
} catch (IOException e) {
close();
throw e;
}
return true;
} 我们一步步分析，由于ClientScanner在构造时，并没有初始化callable成员变量，所以它必定为null，第一部分代码略过，我们以后再讲。
接下来，由于currentRegion也没有被初始化，所以，程序走的是else分支，也就是，将localStartKey设置为scan的startRow。
localStartKey = this.scan.getStartRow();
紧接着，调用getScannerCallable()方法为成员变量callable赋值，入参为行localStartKey和行数nbRows，这个nbRows即为ClientScanner构造时的caching值。方法定义如下：
@InterfaceAudience.Private
protected ScannerCallableWithReplicas getScannerCallable(byte [] localStartKey,
int nbRows) {
// 设置scan的startRow
scan.setStartRow(localStartKey);
// 构造ScannerCallable类型的对象s，将其作为ScannerCallableWithReplicas类型的sr的成员变量currentScannerCallable
ScannerCallable s =
new ScannerCallable(getConnection(), getTable(), scan, this.scanMetrics,
this.rpcControllerFactory);
// 设置caching
s.setCaching(nbRows);
// 构造ScannerCallableWithReplicas类型的对象sr，其成员变量currentScannerCallable为上面的ScannerCallable类型的对象s
ScannerCallableWithReplicas sr = new ScannerCallableWithReplicas(tableName, getConnection(),
s, pool, primaryOperationTimeout, scan,
retries, scannerTimeout, caching, conf, caller);
return sr;
} 首先，把localStartKey设置为scan的startRow，后续每次迭代处理时，我们就能知道scan的起始行。
然后，生成一个ScannerCallable类型的对象s，这个s是要作为ScannerCallableWithReplicas类型的sr的成员变量currentScannerCallable来使用的，实际与Region所在RegionServer通信的正是这个对象，并且，这个ScannerCallable对象中有一个byte[]类型的row成员变量，它会被初始化为scan的startRow，被用来进行Region的定位和其行号的定位。
构造ScannerCallableWithReplicas类型的对象sr并返回，其成员变量currentScannerCallable为上面的ScannerCallable类型的对象s。
getScannerCallable()方法执行完后，紧接着会调用ClientScanner的call()方法，代码如下：
Result[] call(Scan scan, ScannerCallableWithReplicas callable,
RpcRetryingCaller<Result[]> caller, int scannerTimeout)
throws IOException, RuntimeException {
if (Thread.interrupted()) {
throw new InterruptedIOException();
}
// callWithoutRetries is at this layer. Within the ScannerCallableWithReplicas,
// we do a callWithRetries
// caller为RpcRetryingCaller类型
// callable为ScannerCallableWithReplicas类型
return caller.callWithoutRetries(callable, scannerTimeout);
} 之前我们已知道，caller为RpcRetryingCaller类型，它的方法定义如下：
/**
* Call the server once only.
* 仅仅调用server一次
* {@link RetryingCallable} has a strange shape so we can do retrys. Use this invocation if you
* want to do a single call only (A call to {@link RetryingCallable#call(int)} will not likely
* succeed).
* @return an object of type T
* @throws IOException if a remote or network exception occurs
* @throws RuntimeException other unspecified error
*/
public T callWithoutRetries(RetryingCallable<T> callable, int callTimeout)
throws IOException, RuntimeException {
// The code of this method should be shared with withRetries.
this.globalStartTime = EnvironmentEdgeManager.currentTime();
try {
// 先调用prepare()方法，再调用call()方法，超时时间为callTimeout
callable.prepare(false);
return callable.call(callTimeout);
} catch (Throwable t) {
Throwable t2 = translateException(t);
ExceptionUtil.rethrowIfInterrupt(t2);
// It would be nice to clear the location cache here.
if (t2 instanceof IOException) {
throw (IOException)t2;
} else {
throw new RuntimeException(t2);
}
}
} 由代码可以看出，它实际上的处理流程为先调用callable的prepare()方法，再调用callable的call()方法。
接下来，我们转入callable的分析，这个callable为ScannerCallableWithReplicas类型，它的prepare()方法为空，我们重点分析call()方法，代码如下：
@Override
public Result [] call(int timeout) throws IOException {
// If the active replica callable was closed somewhere, invoke the RPC to
// really close it. In the case of regular scanners, this applies. We make couple
// of RPCs to a RegionServer, and when that region is exhausted, we set
// the closed flag. Then an RPC is required to actually close the scanner.
// 第一次调用时，该段代码会被忽略
if (currentScannerCallable != null && currentScannerCallable.closed) {
// For closing we target that exact scanner (and not do replica fallback like in
// the case of normal reads)
if (LOG.isTraceEnabled()) {
LOG.trace("Closing scanner id=" + currentScannerCallable.scannerId);
}
// 最终调用close()方法
Result[] r = currentScannerCallable.call(timeout);
currentScannerCallable = null;
return r;
}
// We need to do the following:
//1. When a scan goes out to a certain replica (default or not), we need to
// continue to hit that until there is a failure. So store the last successfully invoked
// replica
//2. We should close the "losing" scanners (scanners other than the ones we hear back
// from first)
// 我们需要做以下事情：
//1、当一个scan扫描到一个特定的副本（无论默认与否）。我们需要继续执行，直到有一个错误。然后存储上一个成功的副本。
//2、我们应该关闭这个losing scanners
// 根据scan的startRow获取Region位置，使用cache
RegionLocations rl = RpcRetryingCallerWithReadReplicas.getRegionLocations(true,
RegionReplicaUtil.DEFAULT_REPLICA_ID, cConnection, tableName,
currentScannerCallable.getRow());
// allocate a boundedcompletion pool of some multiple of number of replicas.
// We want to accomodate some RPCs for redundant replica scans (but are still in progress)
// 分配一个有界操作的包含一些副本的池
// 构造一个BoundedCompletionService类型的数据结构cs
// cs中包含需要执行的task、已经完成的task和线程池executor，并提供了submit、poll、take等方法
BoundedCompletionService<Pair<Result[], ScannerCallable>> cs =
new BoundedCompletionService<Pair<Result[], ScannerCallable>>(pool, rl.size() * 5);
List<ExecutionException> exceptions = null;
int submitted = 0, completed = 0;
AtomicBoolean done = new AtomicBoolean(false);
replicaSwitched.set(false);
// submit call for the primary replica.
// 提交请求至the primary replica
// 提交task，并添加到outstandingCallables中
// task为用this.currentScannerCallable封装成的RetryingRPC对象
submitted += addCallsForCurrentReplica(cs, rl);
try {
// wait for the timeout to see whether the primary responds back
// 从cs的BlockingQueue<Future<V>>类型的completed中获取任务完成情况
Future<Pair<Result[], ScannerCallable>> f = cs.poll(timeBeforeReplicas,
TimeUnit.MICROSECONDS); // Yes, microseconds
if (f != null) {
Pair<Result[], ScannerCallable> r = f.get();
if (r != null && r.getSecond() != null) {
updateCurrentlyServingReplica(r.getSecond(), r.getFirst(), done, pool);
}
return r == null ? null : r.getFirst(); //great we got a response
}
} catch (ExecutionException e) {
// the primary call failed with RetriesExhaustedException or DoNotRetryIOException
// but the secondaries might still succeed. Continue on the replica RPCs.
exceptions = new ArrayList<ExecutionException>(rl.size());
exceptions.add(e);
completed++;
} catch (CancellationException e) {
throw new InterruptedIOException(e.getMessage());
} catch (InterruptedException e) {
throw new InterruptedIOException(e.getMessage());
}
// submit call for the all of the secondaries at once
// TODO: this may be an overkill for large region replication
submitted += addCallsForOtherReplicas(cs, rl, 0, rl.size() - 1);
try {
while (completed < submitted) {
try {
Future<Pair<Result[], ScannerCallable>> f = cs.take();
Pair<Result[], ScannerCallable> r = f.get();
if (r != null && r.getSecond() != null) {
updateCurrentlyServingReplica(r.getSecond(), r.getFirst(), done, pool);
}
return r == null ? null : r.getFirst(); // great we got an answer
} catch (ExecutionException e) {
// if not cancel or interrupt, wait until all RPC's are done
// one of the tasks failed. Save the exception for later.
if (exceptions == null) exceptions = new ArrayList<ExecutionException>(rl.size());
exceptions.add(e);
completed++;
}
}
} catch (CancellationException e) {
throw new InterruptedIOException(e.getMessage());
} catch (InterruptedException e) {
throw new InterruptedIOException(e.getMessage());
} finally {
// We get there because we were interrupted or because one or more of the
// calls succeeded or failed. In all case, we stop all our tasks.
cs.cancelAll(true);
}
if (exceptions != null && !exceptions.isEmpty()) {
RpcRetryingCallerWithReadReplicas.throwEnrichedException(exceptions.get(0),
retries); // just rethrow the first exception for now.
}
return null; // unreachable
} 又是一个比较长的方法，我们还是一步步分析。
首先，currentScannerCallable虽然已被初始化，但是它的closed还是为false，那么方法的第一块代码会被跳过。
接下来，调用RpcRetryingCallerWithReadReplicas的getRegionLocations()方法，利用cConnection、tableName和currentScannerCallable.getRow()定位Region，得到Region的位置RegionLocations类型的rl。
然后，构造一个BoundedCompletionService类型的数据结构cs，cs中包含需要执行的task、已经完成的task和线程池executor，并提供了submit、poll、take等方法。依靠它，利用线程池完成任务的调度与执行，并同步获取执行结果。
addCallsForCurrentReplica方法就是实现上述逻辑的方法，代码如下：
private int addCallsForCurrentReplica(
BoundedCompletionService<Pair<Result[], ScannerCallable>> cs, RegionLocations rl) {
RetryingRPC retryingOnReplica = new RetryingRPC(currentScannerCallable);
outstandingCallables.add(currentScannerCallable);
cs.submit(retryingOnReplica);
return 1;
} 它将currentScannerCallable封装出成为一个RetryingRPC对象，提交到cs中执行，并添加到数据结构outstandingCallables中。
我们知道，这个currentScannerCallable对象是ScannerCallable类型的，它被线程池调度执行时，依靠call()方法完成业务逻辑，call方法定义如下：
@Override
public Result [] call(int callTimeout) throws IOException {
if (Thread.interrupted()) {
throw new InterruptedIOException();
}
if (closed) {
if (scannerId != -1) {
close();
}
} else {
if (scannerId == -1L) {
this.scannerId = openScanner();
} else {
Result [] rrs = null;
ScanRequest request = null;
try {
incRPCcallsMetrics();
request = RequestConverter.buildScanRequest(scannerId, caching, false, nextCallSeq);
ScanResponse response = null;
PayloadCarryingRpcController controller = controllerFactory.newController();
controller.setPriority(getTableName());
controller.setCallTimeout(callTimeout);
try {
response = getStub().scan(controller, request);
// Client and RS maintain a nextCallSeq number during the scan. Every next() call
// from client to server will increment this number in both sides. Client passes this
// number along with the request and at RS side both the incoming nextCallSeq and its
// nextCallSeq will be matched. In case of a timeout this increment at the client side
// should not happen. If at the server side fetching of next batch of data was over,
// there will be mismatch in the nextCallSeq number. Server will throw
// OutOfOrderScannerNextException and then client will reopen the scanner with startrow
// as the last successfully retrieved row.
// See HBASE-5974
nextCallSeq++;
long timestamp = System.currentTimeMillis();
// Results are returned via controller
CellScanner cellScanner = controller.cellScanner();
rrs = ResponseConverter.getResults(cellScanner, response);
if (logScannerActivity) {
long now = System.currentTimeMillis();
if (now - timestamp > logCutOffLatency) {
int rows = rrs == null ? 0 : rrs.length;
LOG.info("Took " + (now-timestamp) + "ms to fetch "
+ rows + " rows from scanner=" + scannerId);
}
}
// moreResults is only used for the case where a filter exhausts all elements
if (response.hasMoreResults() && !response.getMoreResults()) {
scannerId = -1L;
closed = true;
// Implied that no results were returned back, either.
return null;
}
// moreResultsInRegion explicitly defines when a RS may choose to terminate a batch due
// to size or quantity of results in the response.
if (response.hasMoreResultsInRegion()) {
// Set what the RS said
setHasMoreResultsContext(true);
setServerHasMoreResults(response.getMoreResultsInRegion());
} else {
// Server didn't respond whether it has more results or not.
setHasMoreResultsContext(false);
}
} catch (ServiceException se) {
throw ProtobufUtil.getRemoteException(se);
}
updateResultsMetrics(rrs);
} catch (IOException e) {
if (logScannerActivity) {
LOG.info("Got exception making request " + TextFormat.shortDebugString(request)
+ " to " + getLocation(), e);
}
IOException ioe = e;
if (e instanceof RemoteException) {
ioe = RemoteExceptionHandler.decodeRemoteException((RemoteException)e);
}
if (logScannerActivity && (ioe instanceof UnknownScannerException)) {
try {
HRegionLocation location =
getConnection().relocateRegion(getTableName(), scan.getStartRow());
LOG.info("Scanner=" + scannerId
+ " expired, current region location is " + location.toString());
} catch (Throwable t) {
LOG.info("Failed to relocate region", t);
}
}
// The below convertion of exceptions into DoNotRetryExceptions is a little strange.
// Why not just have these exceptions implment DNRIOE you ask? Well, usually we want
// ServerCallable#withRetries to just retry when it gets these exceptions. In here in
// a scan when doing a next in particular, we want to break out and get the scanner to
// reset itself up again. Throwing a DNRIOE is how we signal this to happen (its ugly,
// yeah and hard to follow and in need of a refactor).
if (ioe instanceof NotServingRegionException) {
// Throw a DNRE so that we break out of cycle of calling NSRE
// when what we need is to open scanner against new location.
// Attach NSRE to signal client that it needs to re-setup scanner.
if (this.scanMetrics != null) {
this.scanMetrics.countOfNSRE.incrementAndGet();
}
throw new DoNotRetryIOException("Resetting the scanner -- see exception cause", ioe);
} else if (ioe instanceof RegionServerStoppedException) {
// Throw a DNRE so that we break out of cycle of the retries and instead go and
// open scanner against new location.
throw new DoNotRetryIOException("Resetting the scanner -- see exception cause", ioe);
} else {
// The outer layers will retry
throw ioe;
}
}
return rrs;
}
}
return null;
} 进入该方法时，closed由于为false，所以执行else分支，并且，由于scannerId未被赋值，所以它最终执行的是openScanner()方法，打开一个Scanner，并将ID赋值给scannerId。
接下来，cs.poll()方法的调用，则是从cs的BlockingQueue<Future<V>>类型的completed中获取任务完成情况l。
然后，调用updateCurrentlyServingReplica()方法，
至此，整个Table.getScanner()方法分析完毕。总结起来，它主要完成了ResultScanner的初始化工作，并未真正请求数据。同时，它还做了以下几件事情：
1、生成一个实现了ResultScanner接口的对象，一般为ClientScanner（其他三个类型以后再分析）；
2、ClientScanner对象中callable被赋值，
接下来，我们继续分析之后的工作。在完成了ResultScanner的初始化后，数据是如何获取的呢？
我们知道，ResultScanner继承自Iterable接口，那么在其实现ClientScanner的抽象父类AbstractClientScanner类中，定义了iterator()方法的实现，主要是通过hasNext()和next()方法来完成遍历的。那我们就来看下ClientScanner的next()方法，代码如下：
@Override
public Result next() throws IOException {
// If the scanner is closed and there's nothing left in the cache, next is a no-op.
if (cache.size() == 0 && this.closed) {
return null;
}
if (cache.size() == 0) {
loadCache();
}
if (cache.size() > 0) {
return cache.poll();
}
// if we exhausted this scanner before calling close, write out the scan metrics
writeScanMetrics();
return null;
} 非常简单，如果scanner的closed为true，并且cache没有数据，则直接返回null，如果closed为false，并且cache没有数据，那么通过loadCache()方法加载数据，然后其他的情况是，只要cache有数据，则直接返回。
在这里，我们就能知道，scan实际上并不是把数据直接全部加载到客户端的，而是在用到的时候才去取。这么做有什么好处呢？
1、避免全部数据尤其是大数据量的情况下全部缓存在客户端，造成客户端的压力；
2、如果等到全部数据都获取后才可以用，客户端的IO、集群网络IO会在数据获取阶段居高不下，延迟较高，而且不如这种类似懒加载的机制，数据边用边效果好的多。
我们继续分析loadCache()方法。
/**
* Contact the servers to load more {@link Result}s in the cache.
*/
protected void loadCache() throws IOException {
Result[] values = null;
long remainingResultSize = maxScannerResultSize;
int countdown = this.caching;
// We need to reset it if it's a new callable that was created
// with a countdown in nextScanner
callable.setCaching(this.caching);
// This flag is set when we want to skip the result returned. We do
// this when we reset scanner because it split under us.
boolean skipFirst = false;
boolean retryAfterOutOfOrderException = true;
// We don't expect that the server will have more results for us if
// it doesn't tell us otherwise. We rely on the size or count of results
boolean serverHasMoreResults = false;
do {
try {
if (skipFirst) {
// Skip only the first row (which was the last row of the last
// already-processed batch).
callable.setCaching(1);
values = call(scan, callable, caller, scannerTimeout);
// When the replica switch happens, we need to do certain operations
// again. The scannercallable will openScanner with the right startkey
// but we need to pick up from there. Bypass the rest of the loop
// and let the catch-up happen in the beginning of the loop as it
// happens for the cases where we see exceptions. Since only openScanner
// would have happened, values would be null
if (values == null && callable.switchedToADifferentReplica()) {
if (this.lastResult != null) { //only skip if there was something read earlier
skipFirst = true;
}
this.currentRegion = callable.getHRegionInfo();
continue;
}
callable.setCaching(this.caching);
skipFirst = false;
}
// Server returns a null values if scanning is to stop. Else,
// returns an empty array if scanning is to go on and we've just
// exhausted current region.
values = call(scan, callable, caller, scannerTimeout);
if (skipFirst && values != null && values.length == 1) {
skipFirst = false; // Already skipped, unset it before scanning again
values = call(scan, callable, caller, scannerTimeout);
}
// When the replica switch happens, we need to do certain operations
// again. The callable will openScanner with the right startkey
// but we need to pick up from there. Bypass the rest of the loop
// and let the catch-up happen in the beginning of the loop as it
// happens for the cases where we see exceptions. Since only openScanner
// would have happened, values would be null
if (values == null && callable.switchedToADifferentReplica()) {
if (this.lastResult != null) { //only skip if there was something read earlier
skipFirst = true;
}
this.currentRegion = callable.getHRegionInfo();
continue;
}
retryAfterOutOfOrderException = true;
} catch (DoNotRetryIOException e) {
// DNRIOEs are thrown to make us break out of retries. Some types of DNRIOEs want us
// to reset the scanner and come back in again.
if (e instanceof UnknownScannerException) {
long timeout = lastNext + scannerTimeout;
// If we are over the timeout, throw this exception to the client wrapped in
// a ScannerTimeoutException. Else, it's because the region moved and we used the old
// id against the new region server; reset the scanner.
if (timeout < System.currentTimeMillis()) {
LOG.info("For hints related to the following exception, please try taking a look at: " +
"https://hbase.apache.org/book.html#trouble.client.scantimeout");
long elapsed = System.currentTimeMillis() - lastNext;
ScannerTimeoutException ex =
new ScannerTimeoutException(elapsed + "ms passed since the last invocation, "
+ "timeout is currently set to " + scannerTimeout);
ex.initCause(e);
throw ex;
}
} else {
// If exception is any but the list below throw it back to the client; else setup
// the scanner and retry.
Throwable cause = e.getCause();
if ((cause != null && cause instanceof NotServingRegionException) ||
(cause != null && cause instanceof RegionServerStoppedException) ||
e instanceof OutOfOrderScannerNextException) {
// Pass
// It is easier writing the if loop test as list of what is allowed rather than
// as a list of what is not allowed... so if in here, it means we do not throw.
} else {
throw e;
}
}
// Else, its signal from depths of ScannerCallable that we need to reset the scanner.
if (this.lastResult != null) {
// The region has moved. We need to open a brand new scanner at
// the new location.
// Reset the startRow to the row we've seen last so that the new
// scanner starts at the correct row. Otherwise we may see previously
// returned rows again.
// (ScannerCallable by now has "relocated" the correct region)
this.scan.setStartRow(this.lastResult.getRow());
// Skip first row returned. We already let it out on previous
// invocation.
skipFirst = true;
}
if (e instanceof OutOfOrderScannerNextException) {
if (retryAfterOutOfOrderException) {
retryAfterOutOfOrderException = false;
} else {
// TODO: Why wrap this in a DNRIOE when it already is a DNRIOE?
throw new DoNotRetryIOException("Failed after retry of " +
"OutOfOrderScannerNextException: was there a rpc timeout?", e);
}
}
// Clear region.
this.currentRegion = null;
// Set this to zero so we don't try and do an rpc and close on remote server when
// the exception we got was UnknownScanner or the Server is going down.
callable = null;
// This continue will take us to while at end of loop where we will set up new scanner.
continue;
}
// 取当前时间
long currentTime = System.currentTimeMillis();
// 更新指标信息
if (this.scanMetrics != null) {
this.scanMetrics.sumOfMillisSecBetweenNexts.addAndGet(currentTime - lastNext);
}
// 更新lastNext
lastNext = currentTime;
// 遍历values，将Result添加到cache中，并累减remainingResultSize、countdown，记录lastResult
if (values != null && values.length > 0) {
for (Result rs : values) {
cache.add(rs);
// We don't make Iterator here
for (Cell cell : rs.rawCells()) {
remainingResultSize -= CellUtil.estimatedHeapSizeOf(cell);
}
countdown--;
this.lastResult = rs;
}
}
// We expect that the server won't have more results for us when we exhaust
// the size (bytes or count) of the results returned. If the server *does* inform us that
// there are more results, we want to avoid possiblyNextScanner(...). Only when we actually
// get results is the moreResults context valid.
if (null != values && values.length > 0 && callable.hasMoreResultsContext()) {
// Only adhere to more server results when we don't have any partialResults
// as it keeps the outer loop logic the same.
serverHasMoreResults = callable.getServerHasMoreResults();
}
// Values == null means server-side filter has determined we must STOP
// !partialResults.isEmpty() means that we are still accumulating partial Results for a
// row. We should not change scanners before we receive all the partial Results for that
// row.
} while (remainingResultSize > 0 && countdown > 0 && !serverHasMoreResults
&& possiblyNextScanner(countdown, values == null));
// 此处countdown为剩余行数，
} 首先，初始化两个变量，remainingResultSize和countdown，这两个变量的含义分别是一次RPC调用获取的数据总大小和总行数，当然，如果remainingResultSize减去已整数行数据大小，比一行数据小，且还有数据要获取，总行数又低于countdown的话，那么我们还是要获取完整的一行数据的，这两个变量随着数据的获取，不断的递减，并参与到是否应该获取数据的逻辑判断，相应代码如下：
// We don't make Iterator here
for (Cell cell : rs.rawCells()) {
remainingResultSize -= CellUtil.estimatedHeapSizeOf(cell);
}
countdown--;
while (remainingResultSize > 0 && countdown > 0 && !serverHasMoreResults
&& possiblyNextScanner(countdown, values == null)); 接下来，进入一个do-while循环，因为skipFirst初始值为false，所以，第一次会直接调用call()方法，结果又到了执行ScannerCallableWithReplicas的call()方法。我们再次分析，同样是先定位Region，获取Region的位置信息，并执行currentScannerCallable即ScannerCallable的call()方法，只不过这次调用，因为有了scannerId，而实际调用RegionServer上的RPC服务，真正去获取数据。代码如下：
Result [] rrs = null;
ScanRequest request = null;
try {
incRPCcallsMetrics();
request = RequestConverter.buildScanRequest(scannerId, caching, false, nextCallSeq);
ScanResponse response = null;
PayloadCarryingRpcController controller = controllerFactory.newController();
controller.setPriority(getTableName());
controller.setCallTimeout(callTimeout);
try {
response = getStub().scan(controller, request);
// Client and RS maintain a nextCallSeq number during the scan. Every next() call
// from client to server will increment this number in both sides. Client passes this
// number along with the request and at RS side both the incoming nextCallSeq and its
// nextCallSeq will be matched. In case of a timeout this increment at the client side
// should not happen. If at the server side fetching of next batch of data was over,
// there will be mismatch in the nextCallSeq number. Server will throw
// OutOfOrderScannerNextException and then client will reopen the scanner with startrow
// as the last successfully retrieved row.
// See HBASE-5974
nextCallSeq++;
long timestamp = System.currentTimeMillis();
// Results are returned via controller
CellScanner cellScanner = controller.cellScanner();
rrs = ResponseConverter.getResults(cellScanner, response);
if (logScannerActivity) {
long now = System.currentTimeMillis();
if (now - timestamp > logCutOffLatency) {
int rows = rrs == null ? 0 : rrs.length;
LOG.info("Took " + (now-timestamp) + "ms to fetch "
+ rows + " rows from scanner=" + scannerId);
}
}
// moreResults is only used for the case where a filter exhausts all elements
if (response.hasMoreResults() && !response.getMoreResults()) {
scannerId = -1L;
closed = true;
// Implied that no results were returned back, either.
return null;
}
// moreResultsInRegion explicitly defines when a RS may choose to terminate a batch due
// to size or quantity of results in the response.
if (response.hasMoreResultsInRegion()) {
// Set what the RS said
setHasMoreResultsContext(true);
setServerHasMoreResults(response.getMoreResultsInRegion());
} else {
// Server didn't respond whether it has more results or not.
setHasMoreResultsContext(false);
}
} catch (ServiceException se) {
throw ProtobufUtil.getRemoteException(se);
}
updateResultsMetrics(rrs);
} catch (IOException e) {
if (logScannerActivity) {
LOG.info("Got exception making request " + TextFormat.shortDebugString(request)
+ " to " + getLocation(), e);
}
IOException ioe = e;
if (e instanceof RemoteException) {
ioe = RemoteExceptionHandler.decodeRemoteException((RemoteException)e);
}
if (logScannerActivity && (ioe instanceof UnknownScannerException)) {
try {
HRegionLocation location =
getConnection().relocateRegion(getTableName(), scan.getStartRow());
LOG.info("Scanner=" + scannerId
+ " expired, current region location is " + location.toString());
} catch (Throwable t) {
LOG.info("Failed to relocate region", t);
}
}
// The below convertion of exceptions into DoNotRetryExceptions is a little strange.
// Why not just have these exceptions implment DNRIOE you ask? Well, usually we want
// ServerCallable#withRetries to just retry when it gets these exceptions. In here in
// a scan when doing a next in particular, we want to break out and get the scanner to
// reset itself up again. Throwing a DNRIOE is how we signal this to happen (its ugly,
// yeah and hard to follow and in need of a refactor).
if (ioe instanceof NotServingRegionException) {
// Throw a DNRE so that we break out of cycle of calling NSRE
// when what we need is to open scanner against new location.
// Attach NSRE to signal client that it needs to re-setup scanner.
if (this.scanMetrics != null) {
this.scanMetrics.countOfNSRE.incrementAndGet();
}
throw new DoNotRetryIOException("Resetting the scanner -- see exception cause", ioe);
} else if (ioe instanceof RegionServerStoppedException) {
// Throw a DNRE so that we break out of cycle of the retries and instead go and
// open scanner against new location.
throw new DoNotRetryIOException("Resetting the scanner -- see exception cause", ioe);
} else {
// The outer layers will retry
throw ioe;
}
}
return rrs; 这段代码的最后，如果不能获取到数据，则将scannerId恢复为-1，并将closed设置为true，以便下次调用时，将上次的callable关闭，并重新生成一个scanner。
后续的逻辑是，如果scanner还有需要获取的数据，重新定位scan的startRow，发送RPC请求至RegionSever，继续获取该Region上的数据，否则，关闭上一个callable，并重新生成一个scanner和callable，并重复以前的逻辑，继续获取数据，直到检索到endRow，或者比endRow大的数据。
还有很多细节和特殊情况没有分析，留待以后慢慢分析吧~~
