Solr建索引时候是有最大的线程数限制的，它由solrconfig.xml的
那么Solr建索引时候是怎么管理线程池的呢，主要是通过ThreadAffinityDocumentsWriterThreadPool来进行管理的，它继承了DocumentsWriterPerThreadPool类。ThreadAffinityDocumentsWriterThreadPool的结构并不复杂，主要的一个函数是getAndLock()。
在建索引时候即updatedocuments时候，Solr先要调用getAndLock去获取ThreadState这个锁。而ThreadState这个锁就是存放在ThreadAffinityDocumentsWriterThreadPool的threadBings这个线程池里面。
首先先看下什么是ThreadState锁，源码如下：
ThreadState是DocumentsWriterPerThreadPool的一个内部类。它包含了一个DocumentsWriterPerThread类的实例以及状态控制，DocumentsWriterPerThread是线程池的一个线程，主要作用是索引的建立。该类比较简单就不详细介绍了。
/**
* {@link ThreadState} references and guards a
* {@link DocumentsWriterPerThread} instance that is used during indexing to
* build a in-memory index segment. {@link ThreadState} also holds all flush
* related per-thread data controlled by {@link DocumentsWriterFlushControl}.
* <p>
* A {@link ThreadState}, its methods and members should only accessed by one
* thread a time. Users must acquire the lock via {@link ThreadState#lock()}
* and release the lock in a finally block via {@link ThreadState#unlock()}
* before accessing the state.
*/
@SuppressWarnings("serial")
final static class ThreadState extends ReentrantLock {
DocumentsWriterPerThread dwpt;
// TODO this should really be part of DocumentsWriterFlushControl
// write access guarded by DocumentsWriterFlushControl
volatile boolean flushPending = false;
// TODO this should really be part of DocumentsWriterFlushControl
// write access guarded by DocumentsWriterFlushControl
long bytesUsed = 0;
// guarded by Reentrant lock
private boolean isActive = true;
ThreadState(DocumentsWriterPerThread dpwt) {
this.dwpt = dpwt;
}
/**
* Resets the internal {@link DocumentsWriterPerThread} with the given one.
* if the given DWPT is <code>null</code> this ThreadState is marked as inactive and should not be used
* for indexing anymore.
* @seeisActive()
*/
private void deactivate() {
assert this.isHeldByCurrentThread();
isActive = false;
reset();
}
private void reset() {
assert this.isHeldByCurrentThread();
this.dwpt = null;
this.bytesUsed = 0;
this.flushPending = false;
}
/**
* Returns <code>true</code> if this ThreadState is still open. This will
* only return <code>false</code> iff the DW has been closed and this
* ThreadState is already checked out for flush.
*/
boolean isActive() {
assert this.isHeldByCurrentThread();
return isActive;
}
boolean isInitialized() {
assert this.isHeldByCurrentThread();
return isActive() && dwpt != null;
}
/**
* Returns the number of currently active bytes in this ThreadState's
* {@link DocumentsWriterPerThread}
*/
public long getBytesUsedPerThread() {
assert this.isHeldByCurrentThread();
// public for FlushPolicy
return bytesUsed;
}
/**
* Returns this {@link ThreadState}s {@link DocumentsWriterPerThread}
*/
public DocumentsWriterPerThread getDocumentsWriterPerThread() {
assert this.isHeldByCurrentThread();
// public for FlushPolicy
return dwpt;
}
/**
* Returns <code>true</code> iff this {@link ThreadState} is marked as flush
* pending otherwise <code>false</code>
*/
public boolean isFlushPending() {
return flushPending;
}
}
/**
* A {@link DocumentsWriterPerThreadPool} implementation that tries to assign an
* indexing thread to the same {@link ThreadState} each time the thread tries to
* obtain a {@link ThreadState}. Once a new {@link ThreadState} is created it is
* associated with the creating thread. Subsequently, if the threads associated
* {@link ThreadState} is not in use it will be associated with the requesting
* thread. Otherwise, if the {@link ThreadState} is used by another thread
* {@link ThreadAffinityDocumentsWriterThreadPool} tries to find the currently
* minimal contended {@link ThreadState}.
*/
class ThreadAffinityDocumentsWriterThreadPool extends DocumentsWriterPerThreadPool {
private Map<Thread, ThreadState> threadBindings = new ConcurrentHashMap<>();
/**
* Creates a new {@link ThreadAffinityDocumentsWriterThreadPool} with a given maximum of {@link ThreadState}s.
*/
public ThreadAffinityDocumentsWriterThreadPool(int maxNumPerThreads) {
super(maxNumPerThreads);
assert getMaxThreadStates() >= 1;
}
@Override
public ThreadState getAndLock(Thread requestingThread, DocumentsWriter documentsWriter) {
ThreadState threadState = threadBindings.get(requestingThread);
if (threadState != null && threadState.tryLock()) {
return threadState;
}
ThreadState minThreadState = null;
/* TODO -- another thread could lock the minThreadState we just got while
we should somehow prevent this. */
// Find the state that has minimum number of threads waiting
minThreadState = minContendedThreadState();
if (minThreadState == null || minThreadState.hasQueuedThreads()) {
final ThreadState newState = newThreadState(); // state is already locked if non-null
if (newState != null) {
assert newState.isHeldByCurrentThread();
threadBindings.put(requestingThread, newState);
return newState;
} else if (minThreadState == null) {
/*
* no new threadState available we just take the minContented one
* This must return a valid thread state since we accessed the
* synced context in newThreadState() above.
*/
minThreadState = minContendedThreadState();
}
}
assert minThreadState != null: "ThreadState is null";
minThreadState.lock();
return minThreadState;
}
@Override
public ThreadAffinityDocumentsWriterThreadPool clone() {
ThreadAffinityDocumentsWriterThreadPool clone = (ThreadAffinityDocumentsWriterThreadPool) super.clone();
clone.threadBindings = new ConcurrentHashMap<>();
return clone;
}
}
再回到ThreadAffinityDocumentWriterThreadPool类。getAndLock的主要流程如下:
1、 请求线程requestingThread需要进行updatedocument操作，它首先会尝试从线程池threadBings获取自身线程的ThreadState锁并尝试去锁它即trylock如果锁成功了，那么它就能再度获取到自身线程的ThreadState，这是最好的一种情况；
2、 如果自身线程的trylock失败，说明该ThreadState已经被别的requestingThread线程抢去，那么请求线程requestingThread只能去线程池threadBings获取别的线程获取的规则是minContendedThreadState(),源码如下所示.；
minContendedThreadState的规则就是遍历所有活跃的ThreadState，如果ThreadState的队列内元素个数最少(即等待这个ThreadState的线程最少)，那么这个ThreadState就是返回的那个ThreadState，即minThreadState.
/**
* Returns the ThreadState with the minimum estimated number of threads
* waiting to acquire its lock or <code>null</code> if no {@link ThreadState}
* is yet visible to the calling thread.
*/
ThreadState minContendedThreadState() {
ThreadState minThreadState = null;
final int limit = numThreadStatesActive;
for (int i = 0; i < limit; i++) {
final ThreadState state = threadStates[i];
if (minThreadState == null || state.getQueueLength() < minThreadState.getQueueLength()) {
minThreadState = state;
}
}
return minThreadState;
}
3、 如果minThreadState==null(一般是第一个获取ThreadState这种情况)或者minThreadState有其他线程在等待(正常情况下都会有线程在等的)，那么requestingThread会去申请新的ThreadState，即从maxIndexingThreads的线程里申请，源码如下；
threadStates是一个ThreadStates的数组，当需要threadBings的ThreadState个数(也就是活跃的线程)小于threadStates的元素个数（maxIndexingThreads）时就能申请到新的ThreadState。
/**
* Returns a new {@link ThreadState} iff any new state is available otherwise
* <code>null</code>.
* <p>
* NOTE: the returned {@link ThreadState} is already locked iff non-
* <code>null</code>.
*
* @return a new {@link ThreadState} iff any new state is available otherwise
* <code>null</code>
*/
synchronized ThreadState newThreadState() {
if (numThreadStatesActive < threadStates.length) {
final ThreadState threadState = threadStates[numThreadStatesActive];
threadState.lock(); // lock so nobody else will get this ThreadState
boolean unlock = true;
try {
if (threadState.isActive()) {
// unreleased thread states are deactivated during DW#close()
numThreadStatesActive++; // increment will publish the ThreadState
assert threadState.dwpt == null;
unlock = false;
return threadState;
}
// unlock since the threadstate is not active anymore - we are closed!
assert assertUnreleasedThreadStatesInactive();
return null;
} finally {
if (unlock) {
// in any case make sure we unlock if we fail
threadState.unlock();
}
}
}
return null;
}
4、 如果minContentedThreadState获取成功，那么threadBings的线程池就会得到更新如果minContentedThreadState获取失败，那么说明threadStates数组以及分配完全，那么请求线程会再去取获取minContentedThreadState；
5、 最后请求线程会去lockminThreadState，如果lock失败就进入休眠，一直等到lock成功这是最不好的一种结果；
最后在源码说道，请求线程在获取minThreadState时候别的线程也有可能获取到该minThreadState，目前来说这是一种缺陷。