Lock/LockFactory類系
綜述：Lucene 用“鎖”的機(jī)制來實(shí)現(xiàn)同一文件夾的互斥訪問：當(dāng)有進(jìn)程訪問需要互斥訪問的文件夾時(shí)，首先查看與之關(guān)聯(lián)的“鎖”是否存在，若存在則拒絕訪問；若不存在，則先上“鎖”，訪問之，最后解 “鎖”。不同的Lock子類，具體的“鎖”實(shí)現(xiàn)方式并不一樣。
1.Lock/LockFactory類系的層次圖
2.部分代碼說明
Lock類
Lock本身是一個(gè)抽象類，它提供了４個(gè)方法，但它僅實(shí)現(xiàn)了obtain(long)這一個(gè)方法，其他三個(gè)留給了它的子類去完成。４個(gè)方法的聲明羅列如下：
public abstract Boolean obtain() throws IOException;
public boolean obtain(long lockWaitTimeout) throws LockObtainFailedException;;
public abstract void release() throws IOException;
public abstract Boolean isLocked();
Lock還提供了兩個(gè)靜態(tài)變量：long LOCK_POLL_INTERVAL（默認(rèn)值為1000ms）和final long LOCK_OBTAIN_WAIT_FOREVER（值為-1） ,前者為試圖獲取“鎖”時(shí)的時(shí)間間隔值，后者為當(dāng)lockWaitTimeout設(shè)置為該值時(shí)，obtain(long)將會(huì)無限期試圖獲取“鎖”。
obtain(long)的功能為在給定的lockWaitTimeout時(shí)間內(nèi)試圖獲取“鎖”，一旦獲取到，則返回；超過時(shí)間則會(huì)拋出異常。其代碼及注釋如下：
1
public boolean obtain(long lockWaitTimeout)
2
throws LockObtainFailedException, IOException 
{
3
failureReason = null;
4 // locked試圖獲取“鎖文件”。obtain()的功能是及時(shí)返回是否能取得“鎖文件”
5 boolean locked = obtain();
6 // 給定參數(shù)值為負(fù)并且不等于-1,則拋出參數(shù)值設(shè)置異常
7 if (lockWaitTimeout < 0 && lockWaitTimeout != LOCK_OBTAIN_WAIT_FOREVER)
8 throw new IllegalArgumentException(
9 "lockWaitTimeout should be LOCK_OBTAIN_WAIT_FOREVER or a non-negative number (got "
10 + lockWaitTimeout + ")");
11 // 設(shè)置最大睡眠次數(shù)
12 long maxSleepCount = lockWaitTimeout / LOCK_POLL_INTERVAL;
13 long sleepCount = 0; // 睡眠次數(shù)累加器
14 // 循環(huán)直到取得“鎖文件”；或者時(shí)間到，則拋出異常
15
while (!locked) {
16 if (lockWaitTimeout != LOCK_OBTAIN_WAIT_FOREVER
17 && sleepCount++ >= maxSleepCount) { // 參數(shù)lockWaitTimeout不為-1且累計(jì)睡眠次數(shù)大于maxSleepCount，拋出異常
18 String reason = "Lock obtain timed out: " + this.toString();
19 if (failureReason != null) {
20 reason += ": " + failureReason;
21
}
22 LockObtainFailedException e = new LockObtainFailedException(
23 reason);
24 if (failureReason != null) {
25 e.initCause(failureReason);
26 }
27 throw e;
28 }
29 try {
30 // 睡眠LOCK_POLL_INTERVAL(默認(rèn)1000ms)時(shí)間
31 Thread.sleep(LOCK_POLL_INTERVAL);
32 } catch (InterruptedException e) {
33 throw new IOException(e.toString());
34 }
35 // 再次試圖獲取“鎖文件”
36 locked = obtain();
37 }
38 // 正常退出，
39 return locked;
40
}
SimpleFSLock類
在SimpleFSLock類中，“鎖”是通過給需要訪問的文件夾另外建立一個(gè)文件的方式來實(shí)現(xiàn)的；查看某文件夾是否被上鎖，你需要做的僅僅是查看下與其相關(guān)的“鎖文件”是否存在；解鎖時(shí)只需刪除“鎖文件”就萬事ＯＫ了。下面是obtain()方法的代碼及注釋：
1 public boolean obtain() throws IOException {
2
3 // Ensure that lockDir exists and is a directory:
4 // 確保lockDir存在并且是文件夾類型
5 if (!lockDir.exists()) {
6 if (!lockDir.mkdirs()) // 如果lockDir不存在，則試圖為其建立新文件夾，建立失敗則拋出異常
7 throw new IOException("Cannot create directory: "
8 + lockDir.getAbsolutePath());
9 } else if (!lockDir.isDirectory()) {
10 // 如果lockDir存在，但不是文件夾，拋出異常
11 throw new IOException(
12 "Found regular file where directory expected: "
13 + lockDir.getAbsolutePath());
14 }
15 // createNewFile成功，返回true; 失敗，false;
16 // 說明：建立成功，也就是說“鎖文件”不存在； 失敗，說明“鎖文件”已經(jīng)存在，也就是說該文件夾已被上鎖
17 return lockFile.createNewFile();
18 }
NativeFSLock類
NativeFSLock與SimpleFSLock有些不同，它的“鎖”用的是“鎖文件”的鎖，說起來很是繞口，其實(shí)它只是給“鎖文件”上一把鎖：在查看某文件夾是否能被訪問時(shí)，首先檢查與此文件夾關(guān)聯(lián)的“鎖文件”的鎖是否被占用，而不像SimpleFSLock僅僅查看與之相連的“鎖文件”是否存在。正因?yàn)槿绱耍鉀Q了如果JVM異常退出時(shí)遺留的“鎖文件”的問題：在SimpleFSLock中，只要“鎖文件”存在，就被人為該文件夾被鎖，而不能被任何其他進(jìn)程訪問。
NativeFSLock額外定義了一個(gè)私有靜態(tài)變量：private static HashSet LOCK_HELD。它用來記錄“鎖文件”的標(biāo)準(zhǔn)路徑名（canonical path），當(dāng)某文件夾的“鎖文件”標(biāo)準(zhǔn)路徑名存在于LOCK_HELD中，且沒有被上鎖,就說明該文件夾可被訪問；否則拒絕訪問。在解鎖時(shí)，需要從LOCK_HELD中刪除“鎖文件”的標(biāo)準(zhǔn)路徑名，刪除“鎖文件”。
NativeFSLock的代碼中包含了很多對(duì)于異常的處理，使得程序看起來很是費(fèi)解。
obtain()的主要代碼及注釋如下：
1 public synchronized boolean obtain() throws IOException { // 該方法被設(shè)置為同步訪問
2 // isLocked()為true說明“鎖文件”已被上鎖，正在被使用中
3 if (isLocked()) {
4 // Our instance is already locked:
5 return false;
6 }
7
8 // Ensure that lockDir exists and is a directory.
9 if (!lockDir.exists()) {
10 if (!lockDir.mkdirs())
11 throw new IOException("Cannot create directory: "
12 + lockDir.getAbsolutePath());
13 } else if (!lockDir.isDirectory()) {
14 throw new IOException(
15 "Found regular file where directory expected: "
16 + lockDir.getAbsolutePath());
17 }
18
19 String canonicalPath = path.getCanonicalPath();
20
21 boolean markedHeld = false; //標(biāo)記在LOCK_HELD中是否存在某“鎖文件”的路徑名
22
23 try {
24
25 // Make sure nobody else in-process has this lock held
26 // already, and, mark it held if not:
27
28 synchronized (LOCK_HELD) { // 設(shè)置LOCK_HELD的同步訪問
29 if (LOCK_HELD.contains(canonicalPath)) { // 如果標(biāo)準(zhǔn)路徑存在于LOCK_HELD中，說明該文件被上鎖或正在被上鎖，返回false
30 // Someone else in this JVM already has the lock:
31 return false;
32 } else {
33 // This "reserves" the fact that we are the one
34 // thread trying to obtain this lock, so we own
35 // the only instance of a channel against this
36 // file:
37 LOCK_HELD.add(canonicalPath); // 添加路徑名到LOCK_HELD中
38 markedHeld = true; // 設(shè)置為true
39 }
40 }
41
42 try {
43 // 建立“鎖文件”
44 f = new RandomAccessFile(path, "rw");
45 } catch (IOException e) {
46 // On Windows, we can get intermittant "Access
47 // Denied" here. So, we treat this as failure to
48 // acquire the lock, but, store the reason in case
49 // there is in fact a real error case.
50 failureReason = e;
51 f = null; // 建立失敗，則ｆ= null
52 }
53
54 if (f != null) {
55 try {
56 // 獲取“鎖文件”的通道
57 channel = f.getChannel();
58 try {
59 // 給“鎖文件”上鎖
60 lock = channel.tryLock();
61 } catch (IOException e) {
62 // At least on OS X, we will sometimes get an
63 // intermittant "Permission Denied" IOException,
64 // which seems to simply mean "you failed to get
65 // the lock". But other IOExceptions could be
66 // "permanent" (eg, locking is not supported via
67 // the filesystem). So, we record the failure
68 // reason here; the timeout obtain (usually the
69 // one calling us) will use this as "root cause"
70 // if it fails to get the lock.
71 failureReason = e;
72 } finally {
73 if (lock == null) { // 如果沒有取得鎖，需關(guān)閉通道并設(shè)置其為null
74 try {
75 channel.close(); //關(guān)閉通道
76 } finally {
77 channel = null; //設(shè)置為null
78 }
79 }
80 }
81 } finally {
82 if (channel == null) { // 如果通道獲取失敗或者上鎖異常，關(guān)閉“鎖文件”
83 try {
84 f.close(); // 關(guān)閉“鎖文件”
85 } finally {
86 f = null;
87 }
88 }
89 }
90 }
91
92 } finally {
93 // markedHeld為ture，但isLocked()為false,說明上鎖途中出現(xiàn)異常
94 // 需刪除“鎖文件”的路徑名
95 if (markedHeld && !isLocked()) {
96 synchronized (LOCK_HELD) { // 注意同步訪問LOCK_HELD
97 if (LOCK_HELD.contains(canonicalPath)) {
98 LOCK_HELD.remove(canonicalPath);
99 }
100 }
101 }
102 }
103 // 經(jīng)過以上過程，若成功上鎖則isLock()為true；反之，false
104 return isLocked();
105 }