本文小编为大家详细介绍“Lucene索引删除策略源码分析”，内容详细，步骤清晰，细节处理妥当，希望这篇“Lucene索引删除策略源码分析”文章能帮助大家解决疑惑，下面跟着小编的思路慢慢深入，一起来学习新知识吧。
IndexCommit
Lucene中，需要持久化的索引信息都要进行commit操作，然后会生成一个segments_N的索引文件记录此次commit相关的索引信息。
一次commit生成segments_N之后，就对应了一个IndexCommit，IndexCommit只是一个接口，它定义了可以从IndexCommit中获取哪些信息：
public abstract class IndexCommit implements Comparable<IndexCommit> { // commit对应的segments_N public abstract String getSegmentsFileName(); // commit关联的所有的索引文件 public abstract Collection<String> getFileNames() throws IOException; // 索引所在的Directory public abstract Directory getDirectory(); // 删除commit，后面会看到，删除其实减少commit关联的索引文件的引用计数 public abstract void delete(); // commit是否被删除了 public abstract boolean isDeleted(); // commit关联了几个segment public abstract int getSegmentCount(); // segments_N文件中的N public abstract long getGeneration(); // commit可以记录一些用户自定义的信息 public abstract Map<String, String> getUserData() throws IOException; // 用来读取commit对应的索引数据 StandardDirectoryReader getReader() { return null; }}IndexCommit有三个实现类：
CommitPoint
ReaderCommit
SnapshotCommitPoint
这个三个实现类都有对应的使用场景，在用到的时候我会再详细介绍，本文中会涉及到SnapshotCommitPoint，后面会详细介绍它。
IndexDeletionPolicy
在索引的生命周期中，可以有多次的commit操作，因此也会生成多个segments_N文件，对于这些文件是否要保留还是删除，lucene中是通过IndexDeletionPolicy来管理的。我们先来看下IndexDeletionPolicy的接口定义：
public abstract class IndexDeletionPolicy { protected IndexDeletionPolicy() {} // 重新打开索引的时候，对所有commit的处理 public abstract void onInit(List<? extends IndexCommit> commits) throws IOException; // 有新提交时对所有commit的处理 public abstract void onCommit(List<? extends IndexCommit> commits) throws IOException;}从上面我可以看到，索引的删除策略其实只在两个地方进行应用，一个是加载索引的时候，打开一个旧索引时，根据当前设置的IndexDeletionPolicy进行处理。另一个是有新的commit产生时，借这个机会处理所有的commit。Lucene中提供的索引删除策略一共有四种，不过可以分为三类：
NoDeletionPolicy
NoDeletionPolicy索引删除策略就是保留所有的commit信息，效果就是你有多少次commit就多少个segments_N文件，看个例子：
public class DeletionPolicyTest { private static final Random RANDOM = new Random(); public static void main(String[] args) throws IOException { Directory directory = FSDirectory.open(new File("D:\\code\\lucene-9.1.0-learning\\data").toPath()); WhitespaceAnalyzer analyzer = new WhitespaceAnalyzer(); IndexWriterConfig indexWriterConfig = new IndexWriterConfig(analyzer); indexWriterConfig.setUseCompoundFile(true); indexWriterConfig.setIndexDeletionPolicy(NoDeletionPolicy.INSTANCE); IndexWriter indexWriter = new IndexWriter(directory, indexWriterConfig); indexWriter.addDocument(getDoc(RANDOM.nextInt(10000),RANDOM.nextInt(10000))); // 第一次commit，生成segments_1 indexWriter.commit(); indexWriter.addDocument(getDoc(RANDOM.nextInt(10000),RANDOM.nextInt(10000))); // 第二次commit，生成segments_2 indexWriter.commit(); indexWriter.close(); } private static Document getDoc(int... point) { Document doc = new Document(); IntPoint intPoint = new IntPoint("point", point); doc.add(intPoint); return doc; }}上面的例子中有两次commit，下图是NoDeletionPolicy策略进行了两次commit的索引目录结构，可以看到生成了两个segments_N文件：
NoDeletionPolicy的代码实现非常简单，单例实现，并且在onCommit和onInit的时候都是空操作：
public final class NoDeletionPolicy extends IndexDeletionPolicy { public static final IndexDeletionPolicy INSTANCE = new NoDeletionPolicy(); private NoDeletionPolicy() { } public void onCommit(List<? extends IndexCommit> commits) {} public void onInit(List<? extends IndexCommit> commits) {}}KeepOnlyLastCommitDeletionPolicy
KeepOnlyLastCommitDeletionPolicy是Lucene默认的索引删除策略，只保留最新的一次commit，从索引目录看不管执行多少次commit只保留了N最大的segments_N文件，下图是KeepOnlyLastCommitDeletionPolicy策略进行了两次commit的结果，KeepOnlyLastCommitDeletionPolicy删除策略只保留了segments_2。把上面示例代码中的删除策略替换成KeepOnlyLastCommitDeletionPolicy，即可得到，注意需要先清空索引目录：
KeepOnlyLastCommitDeletionPolicy代码实现也比较简单，除了最后一个commit之外，其他的commit都删除：
public final class KeepOnlyLastCommitDeletionPolicy extends IndexDeletionPolicy { public KeepOnlyLastCommitDeletionPolicy() {} public void onInit(List<? extends IndexCommit> commits) { onCommit(commits); } // commits是从旧到新排序的 public void onCommit(List<? extends IndexCommit> commits) { // 只保留最新的一个 int size = commits.size(); for (int i = 0; i < size - 1; i++) { commits.get(i).delete(); } }}两个快照相关的删除策略
快照相关的删除策略有两个，SnapshotDeletionPolicy和PersistentSnapshotDeletionPolicy，分别对应了不可持久化和可持久化的模式。不管是SnapshotDeletionPolicy还是PersistentSnapshotDeletionPolicy，他们都封装了其他的IndexDeletionPolicy来执行删除策略，他们只是提供了为当前最新的commit生成快照的能力。只要快照存在，则跟快照相关的所有索引文件都会被无条件保留。
SnapshotDeletionPolicy
例子
public class SnapshotDeletionPolicyTest { private static final Random RANDOM = new Random(); public static void main(String[] args) throws IOException, InterruptedException { Directory directory = FSDirectory.open(new File("D:\\code\\lucene-9.1.0-learning\\data").toPath()); WhitespaceAnalyzer analyzer = new WhitespaceAnalyzer(); IndexWriterConfig indexWriterConfig = new IndexWriterConfig(analyzer); indexWriterConfig.setUseCompoundFile(true); SnapshotDeletionPolicy snapshotDeletionPolicy = new SnapshotDeletionPolicy(new KeepOnlyLastCommitDeletionPolicy()); indexWriterConfig.setIndexDeletionPolicy(snapshotDeletionPolicy); IndexWriter indexWriter = new IndexWriter(directory, indexWriterConfig); indexWriter.addDocument(getDoc(RANDOM.nextInt(10000),RANDOM.nextInt(10000))); // 第一次commit，生成segments_1 indexWriter.commit(); indexWriter.addDocument(getDoc(RANDOM.nextInt(10000),RANDOM.nextInt(10000))); // 第二次commit，生成segments_2 indexWriter.commit(); // segments_2当做快照，无条件保留 snapshotDeletionPolicy.snapshot(); indexWriter.addDocument(getDoc(RANDOM.nextInt(10000),RANDOM.nextInt(10000))); // 第三次commit，生成segments_3 indexWriter.commit(); indexWriter.close(); } private static Document getDoc(int... point) { Document doc = new Document(); IntPoint intPoint = new IntPoint("point", point); doc.add(intPoint); return doc; }}在上面的例子中，我们使用SnapshotDeletionPolicy，SnapshotDeletionPolicy底层封装的是KeepOnlyLastCommitDeletionPolicy，我们进行了三次commit，理论上KeepOnlyLastCommitDeletionPolicy只会保留最后一次，但是因为我们对第一次的commit进行了快照，所以第一次commit也被保留了：
接下来我们看看SnapshotDeletionPolicy是怎么实现。SnapshotDeletionPolicy保证生成快照的commit不会被删除的原理就是引用计数，SnapshotDeletionPolicy会记录每个commit生成快照的次数，在删除的时候，只会删除引用计数为0的commit。
成员变量
// key是IndexCommit的generation，value是对应的IndexCommit有多少个快照 // 需要注意的是，有被快照引用的才会记录在refCounts中，也就是只要被记录在refCounts中，引用次数至少是1 protected final Map<Long, Integer> refCounts = new HashMap<>(); // key是IndexCommit的generation，value是对应的IndexCommit protected final Map<Long, IndexCommit> indexCommits = new HashMap<>(); // SnapshotDeletionPolicy只是增加了支持快照的功能，删除的逻辑是由primary参数对应的删除策略提供的 private final IndexDeletionPolicy primary; // 最近一次提交的commit，只会对这个IndexCommit生成快照 protected IndexCommit lastCommit; // 是否初始化的标记，实例化后，必须先调用onInit方法 private boolean initCalled;生成快照
生成快照只会对当前最新的一个commit进行快照：
public synchronized IndexCommit snapshot() throws IOException { if (!initCalled) { throw new IllegalStateException( "this instance is not being used by IndexWriter; be sure to use the instance returned from writer.getConfig().getIndexDeletionPolicy()"); } if (lastCommit == null) { throw new IllegalStateException("No index commit to snapshot"); } // 新增lastCommit的引用计数 incRef(lastCommit); return lastCommit; } protected synchronized void incRef(IndexCommit ic) { long gen = ic.getGeneration(); Integer refCount = refCounts.get(gen); int refCountInt; if (refCount == null) { // 第一次被引用 indexCommits.put(gen, lastCommit); refCountInt = 0; } else { refCountInt = refCount.intValue(); } // 引用计数加+1 refCounts.put(gen, refCountInt + 1); }释放指定的快照
public synchronized void release(IndexCommit commit) throws IOException { long gen = commit.getGeneration(); releaseGen(gen);}protected void releaseGen(long gen) throws IOException { if (!initCalled) { throw new IllegalStateException( "this instance is not being used by IndexWriter; be sure to use the instance returned from writer.getConfig().getIndexDeletionPolicy()"); } Integer refCount = refCounts.get(gen); if (refCount == null) { throw new IllegalArgumentException("commit gen=" + gen + " is not currently snapshotted"); } int refCountInt = refCount.intValue(); assert refCountInt > 0; refCountInt--; if (refCountInt == 0) { // 引用计数为0，直接从refCounts中移除 refCounts.remove(gen); indexCommits.remove(gen); } else { refCounts.put(gen, refCountInt); }}删除commit
public synchronized void onCommit(List<? extends IndexCommit> commits) throws IOException { // 把commits中的所有IndexCommit都封装成SnapshotCommitPoint，再使用primary执行onCommit方法 primary.onCommit(wrapCommits(commits)); // 更新最新的commit lastCommit = commits.get(commits.size() - 1); } @Override public synchronized void onInit(List<? extends IndexCommit> commits) throws IOException { // 设置初始化的标记 initCalled = true; primary.onInit(wrapCommits(commits)); for (IndexCommit commit : commits) { if (refCounts.containsKey(commit.getGeneration())) { indexCommits.put(commit.getGeneration(), commit); } } if (!commits.isEmpty()) { lastCommit = commits.get(commits.size() - 1); } } private List<IndexCommit> wrapCommits(List<? extends IndexCommit> commits) { List<IndexCommit> wrappedCommits = new ArrayList<>(commits.size()); for (IndexCommit ic : commits) { // 把IndexCommit都封装成 SnapshotCommitPoint wrappedCommits.add(new SnapshotCommitPoint(ic)); } return wrappedCommits; }前面我们列出了SnapshotCommitPoint是IndexCommit的一个实现类，但是没有详细介绍，SnapshotCommitPoint除了能够提供IndexCommit接口所提供的信息之外，最核心的是在删除的时候，会先判断IndexCommit是否被快照引用，只有没有任何快照引用的IndexCommit才能删除：
public void delete() { synchronized (SnapshotDeletionPolicy.this) { if (!refCounts.containsKey(cp.getGeneration())) { cp.delete(); } }}存在的问题
需要注意的是SnapshotDeletionPolicy的快照信息是没有持久化，我们重新打开SnapshotDeletionPolicyTest例子中生成的索引：
public class SnapshotDeletionPolicyTest2 { public static void main(String[] args) throws IOException, InterruptedException { Directory directory = FSDirectory.open(new File("D:\\code\\lucene-9.1.0-learning\\data").toPath()); WhitespaceAnalyzer analyzer = new WhitespaceAnalyzer(); IndexWriterConfig indexWriterConfig = new IndexWriterConfig(analyzer); indexWriterConfig.setUseCompoundFile(true); SnapshotDeletionPolicy snapshotDeletionPolicy = new SnapshotDeletionPolicy(new KeepOnlyLastCommitDeletionPolicy()); indexWriterConfig.setIndexDeletionPolicy(snapshotDeletionPolicy); // 重新打开索引 IndexWriter indexWriter = new IndexWriter(directory, indexWriterConfig); indexWriter.close(); }}可以发现segments_1被删除了，因为没有持久化快照信息，所以根据KeepOnlyLastCommitDeletionPolicy的删除策略，只保留了最新的一个commit：
PersistentSnapshotDeletionPolicy
例子
PersistentSnapshotDeletionPolicy主要是为了解决SnapshotDeletionPolicy无法持久化的问题。PersistentSnapshotDeletionPolicy持久化的时候会生成snapshots_N的索引文件，我们看个例子：
public class PersistentSnapshotDeletionPolicyTest { private static final Random RANDOM = new Random(); public static void main(String[] args) throws IOException, InterruptedException { Directory directory = FSDirectory.open(new File("D:\\code\\lucene-9.1.0-learning\\data").toPath()); WhitespaceAnalyzer analyzer = new WhitespaceAnalyzer(); IndexWriterConfig indexWriterConfig = new IndexWriterConfig(analyzer); indexWriterConfig.setUseCompoundFile(true); PersistentSnapshotDeletionPolicy persistentSnapshotDeletionPolicy = new PersistentSnapshotDeletionPolicy(new KeepOnlyLastCommitDeletionPolicy(), directory); indexWriterConfig.setIndexDeletionPolicy(persistentSnapshotDeletionPolicy); IndexWriter indexWriter = new IndexWriter(directory, indexWriterConfig); indexWriter.addDocument(getDoc(RANDOM.nextInt(10000),RANDOM.nextInt(10000))); // 第一次commit，生成segments_1 indexWriter.commit(); // segments_1当做快照，无条件保留 persistentSnapshotDeletionPolicy.snapshot(); indexWriter.addDocument(getDoc(RANDOM.nextInt(10000),RANDOM.nextInt(10000))); // 第二次commit，生成segments_2 indexWriter.commit(); indexWriter.addDocument(getDoc(RANDOM.nextInt(10000),RANDOM.nextInt(10000))); // 第三次commit，生成segments_3 indexWriter.commit(); indexWriter.close(); } private static Document getDoc(int... point) { Document doc = new Document(); IntPoint intPoint = new IntPoint("point", point); doc.add(intPoint); return doc; }}上面的例子和我们在介绍SnapshotDeletionPolicy的时候逻辑一样，只是把SnapshotDeletionPolicy换成了PersistentSnapshotDeletionPolicy，我们看结果：
从上面结果图中可以看到，segments_1和segments_3同样被保留了，但是多了一个持久化的快照信息的文件snapshots_0，有了这个文件，索引重新打开的时候就可以恢复快照信息，segments_1还是会被保留，用下面的例子我们重新打开索引，可以发现segments_1还是被保留了：
public class PersistentSnapshotDeletionPolicyTest2 { public static void main(String[] args) throws IOException, InterruptedException { Directory directory = FSDirectory.open(new File("D:\\code\\lucene-9.1.0-learning\\data").toPath()); WhitespaceAnalyzer analyzer = new WhitespaceAnalyzer(); IndexWriterConfig indexWriterConfig = new IndexWriterConfig(analyzer); indexWriterConfig.setUseCompoundFile(true); PersistentSnapshotDeletionPolicy persistentSnapshotDeletionPolicy = new PersistentSnapshotDeletionPolicy(new KeepOnlyLastCommitDeletionPolicy(), directory); indexWriterConfig.setIndexDeletionPolicy(persistentSnapshotDeletionPolicy); IndexWriter indexWriter = new IndexWriter(directory, indexWriterConfig); indexWriter.close(); }}接下来我们看看PersistentSnapshotDeletionPolicy的实现，主要就是持久化和恢复快照信息的逻辑。
成员变量
// 持久化快照信息的文件名snapshots_N中的N，从0开始 private long nextWriteGen; // 持久化的文件所在的目录 private final Directory dir;构造函数
public PersistentSnapshotDeletionPolicy(IndexDeletionPolicy primary, Directory dir) throws IOException { this(primary, dir, OpenMode.CREATE_OR_APPEND); } public PersistentSnapshotDeletionPolicy(IndexDeletionPolicy primary, Directory dir, OpenMode mode) throws IOException { super(primary); this.dir = dir; if (mode == OpenMode.CREATE) { // 新建索引的模式，则需要清除所有的快照信息，索引模式以后再介绍 clearPriorSnapshots(); } // 加载快照信息 loadPriorSnapshots(); if (mode == OpenMode.APPEND && nextWriteGen == 0) { throw new IllegalStateException("no snapshots stored in this directory"); } }生成快照
public synchronized IndexCommit snapshot() throws IOException { // 使用SnapshotDeletionPolicy来生成快照 IndexCommit ic = super.snapshot(); // 标记持久化是否成功，不成功的话需要删除快照 boolean success = false; try { // 持久化最新的快照信息 persist(); success = true; } finally { if (!success) { // 持久化失败，删除快照 try { super.release(ic); } catch ( @SuppressWarnings("unused") Exception e) { // Suppress so we keep throwing original exception } } } return ic;}释放快照
public synchronized void release(IndexCommit commit) throws IOException { // 使用SnapshotDeletionPolicy来释放快照 super.release(commit); // 持久化快照信息是否成功 boolean success = false; try { // 持久化最新的快照信息 persist(); success = true; } finally { if (!success) { // 持久化失败，重新加回快照信息 try { incRef(commit); } catch ( @SuppressWarnings("unused") Exception e) { // Suppress so we keep throwing original exception } } }}持久化快照信息
private synchronized void persist() throws IOException { // 快照文件名 String fileName = SNAPSHOTS_PREFIX + nextWriteGen; boolean success = false; try (IndexOutput out = dir.createOutput(fileName, IOContext.DEFAULT)) { CodecUtil.writeHeader(out, CODEC_NAME, VERSION_CURRENT); out.writeVInt(refCounts.size()); for (Entry<Long, Integer> ent : refCounts.entrySet()) { // 持久化所有的引用信息 out.writeVLong(ent.getKey()); out.writeVInt(ent.getValue()); } success = true; } finally { if (!success) { IOUtils.deleteFilesIgnoringExceptions(dir, fileName); } } dir.sync(Collections.singletonList(fileName)); if (nextWriteGen > 0) { String lastSaveFile = SNAPSHOTS_PREFIX + (nextWriteGen - 1); // 删除前一个快照文件，因为每次持久化都是把当前的快照信息全量持久化，所以只需要保留最新的一个就可以 // 这里有可能删除失败，所以在启动加载的时候会再次尝试把旧版本的文件都删掉 IOUtils.deleteFilesIgnoringExceptions(dir, lastSaveFile); } nextWriteGen++;}加载快照信息
private synchronized void loadPriorSnapshots() throws IOException { long genLoaded = -1; IOException ioe = null; List<String> snapshotFiles = new ArrayList<>(); for (String file : dir.listAll()) { if (file.startsWith(SNAPSHOTS_PREFIX)) { // 找到快照文件 long gen = Long.parseLong(file.substring(SNAPSHOTS_PREFIX.length())); if (genLoaded == -1 || gen > genLoaded) { // 找到gen最大的快照文件 snapshotFiles.add(file); Map<Long, Integer> m = new HashMap<>(); IndexInput in = dir.openInput(file, IOContext.DEFAULT); try { CodecUtil.checkHeader(in, CODEC_NAME, VERSION_START, VERSION_START); int count = in.readVInt(); for (int i = 0; i < count; i++) { long commitGen = in.readVLong(); int refCount = in.readVInt(); m.put(commitGen, refCount); } } catch (IOException ioe2) { // 保存第一个捕获到的异常 if (ioe == null) { ioe = ioe2; } } finally { in.close(); } genLoaded = gen; // 清除旧数据 refCounts.clear(); // 保留最新的 refCounts.putAll(m); } } } if (genLoaded == -1) { // 没有加载快照文件 if (ioe != null) { // 加载过程中捕获到异常了，直接抛出 throw ioe; } } else { // 把旧版本的快照文件都删掉 if (snapshotFiles.size() > 1) { String curFileName = SNAPSHOTS_PREFIX + genLoaded; for (String file : snapshotFiles) { if (!curFileName.equals(file)) { IOUtils.deleteFilesIgnoringExceptions(dir, file); } } } nextWriteGen = 1 + genLoaded; }}读到这里，这篇“Lucene索引删除策略源码分析”文章已经介绍完毕，想要掌握这篇文章的知识点还需要大家自己动手实践使用过才能领会，如果想了解更多相关内容的文章，欢迎关注编程网行业资讯频道。
