解决 java.util.prefs.BackingStoreException 报错问题

启动tomcat后，发现有个warnning

信息如下

java.util.prefs.FileSystemPreferences checkLockFile0ErrorCode 

Warning: Could not locksystem prefs. Unix error code
0. 
java.util.prefs.FileSystemPreferences syncWorld 
Warning: Couldn't flush system prefs: java.util.prefs.BackingStoreException: Couldn't get file lock.

这个warnning每半个分钟出现一次，虽然不是什么特别严重的问题，但是我还是决定看看。

首先找到java.util.prefs.FileSystemPreferences syncWorld  这个方法

private static void syncWorld() {
/*
* Synchronization necessary because userRoot and systemRoot are
* lazily initialized.
*/
Preferences userRt;
Preferences systemRt;
synchronized(FileSystemPreferences.class) {
userRt = userRoot;
systemRt = systemRoot;
}

try {
if (userRt != null)
userRt.flush();
} catch(BackingStoreException e) {
getLogger().warning("Couldn't flush user prefs: " + e);
}

try {
if (systemRt != null)
systemRt.flush();
} catch(BackingStoreException e) {
getLogger().warning("Couldn't flush system prefs: " + e);
}
}


看起来应该是flush 这个方法的抛出异常了

再往里跟

public void flush() throws BackingStoreException {
if (isRemoved())
return;
sync();
}

应该是sync的里面报错

public synchronized void sync() throws BackingStoreException {
boolean userNode = isUserNode();
boolean shared;

if (userNode) {
shared = false; /* use exclusive lock for user prefs */
} else {
/* if can write to system root, use exclusive lock.
otherwise use shared lock. */
shared = !isSystemRootWritable;
}
synchronized (isUserNode()? userLockFile:systemLockFile) {
if (!lockFile(shared))
throw(new BackingStoreException("Couldn't get file lock."));

最后一行的错误信息在日志里打出来了

这里看起来应该是lockfile返回的是false，于是进去

private boolean lockFile(boolean shared) throws SecurityException{
boolean usernode = isUserNode();
int[] result;
int errorCode = 0;
File lockFile = (usernode ? userLockFile : systemLockFile);
long sleepTime = INIT_SLEEP_TIME;
for (int i = 0; i < MAX_ATTEMPTS; i++) {
try {
int perm = (usernode? USER_READ_WRITE: USER_RW_ALL_READ);
result = lockFile0(lockFile.getCanonicalPath(), perm, shared);

errorCode = result[ERROR_CODE];
if (result[LOCK_HANDLE] != 0) {
if (usernode) {
userRootLockHandle = result[LOCK_HANDLE];
} else {
systemRootLockHandle = result[LOCK_HANDLE];
}
return true;
}
} catch(IOException e) {
// // If at first, you don't succeed...
}

try {
Thread.sleep(sleepTime);
} catch(InterruptedException e) {
checkLockFile0ErrorCode(errorCode);
return false;
}
sleepTime *= 2;
}
checkLockFile0ErrorCode(errorCode);
return false;
}

这个方法，根据要锁的是用户文件还是系统文件，去锁对应的文件，最多重试到最大次数之后返回false

因为报的是系统文件无法锁住，我就看了一下系统文件在哪里赋值

private static void setupSystemRoot() {
AccessController.doPrivileged(new PrivilegedAction<Void>() {
public Void run() {
String systemPrefsDirName =
System.getProperty("java.util.prefs.systemRoot","/etc/.java");
systemRootDir =
new File(systemPrefsDirName, ".systemPrefs");
// Attempt to create root dir if it does not yet exist.
if (!systemRootDir.exists()) {
// system root does not exist in /etc/.java
// Switching to java.home
systemRootDir =
new File(System.getProperty("java.home"),
".systemPrefs");
if (!systemRootDir.exists()) {
if (systemRootDir.mkdirs()) {
getLogger().info(
"Created system preferences directory "
+ "in java.home.");
try {
chmod(systemRootDir.getCanonicalPath(),
USER_RWX_ALL_RX);
} catch (IOException e) {
}
} else {
getLogger().warning("Could not create "
+ "system preferences directory. System "
+ "preferences are unusable.");
}
}
}
isSystemRootWritable = systemRootDir.canWrite();
systemLockFile = new File(systemRootDir, ".system.lock");
systemRootModFile =
new File (systemRootDir,".systemRootModFile");
if (!systemRootModFile.exists() && isSystemRootWritable)
try {
// create if does not exist.
systemRootModFile.createNewFile();
int result = chmod(systemRootModFile.getCanonicalPath(),
USER_RW_ALL_READ);
if (result !=0)
getLogger().warning("Chmod failed on " +
systemRootModFile.getCanonicalPath() +
" Unix error code " + result);
} catch (IOException e) { getLogger().warning(e.toString());
}
systemRootModTime = systemRootModFile.lastModified();
return null;
}
});
}

看到这里明白了，如果jvm没有设systemroot的话，那是默认etc下面的那个.java文件夹，如果这个文件夹存在，就用这个，如果不存在，那就用java.home的这个

我去我的etc下面看了一下，果然有.java这个隐藏文件夹，我给他干掉之后就没有那个警告了。或者设定上systemroot 这个参数
http://www-01.ibm.com/support/docview.wss?uid=swg21515420 这篇文章讲的是user pref cannot lock这个问题