序
本文主要研究一下logback的FileAppender
FileAppender
ch/qos/logback/core/FileAppender.java
代码语言：javascript复制public class FileAppender<E> extends OutputStreamAppender<E> {
public static final long DEFAULT_BUFFER_SIZE = 8192;
static protected String COLLISION_WITH_EARLIER_APPENDER_URL = CODES_URL "#earlier_fa_collision";
/**
* Append to or truncate the file? The default value for this variable is
* <code>true</code>, meaning that by default a <code>FileAppender</code> will
* append to an existing file and not truncate it.
*/
protected boolean append = true;
/**
* The name of the active log file.
*/
protected String fileName = null;
private boolean prudent = false;
private FileSize bufferSize = new FileSize(DEFAULT_BUFFER_SIZE);
//......
} FileAppender继承了OutputStreamAppender，它定义了append、prudent、bufferSize属性
start
代码语言：javascript复制 public void start() {
int errors = 0;
if (getFile() != null) {
addInfo("File property is set to [" fileName "]");
if (prudent) {
if (!isAppend()) {
setAppend(true);
addWarn("Setting "Append" property to true on account of "Prudent" mode");
}
}
if (checkForFileCollisionInPreviousFileAppenders()) {
addError("Collisions detected with FileAppender/RollingAppender instances defined earlier. Aborting.");
addError(MORE_INFO_PREFIX COLLISION_WITH_EARLIER_APPENDER_URL);
errors ;
} else {
// file should be opened only if collision free
try {
openFile(getFile());
} catch (java.io.IOException e) {
errors ;
addError("openFile(" fileName "," append ") call failed.", e);
}
}
} else {
errors ;
addError(""File" property not set for appender named [" name "].");
}
if (errors == 0) {
super.start();
}
}start方法要求fileName必须有值，在prudent模式下会强制开启append；另外start的时候会执行checkForFileCollisionInPreviousFileAppenders判断是否有冲突，没有冲突则执行openFile方法
checkForFileCollisionInPreviousFileAppenders
代码语言：javascript复制 protected boolean checkForFileCollisionInPreviousFileAppenders() {
boolean collisionsDetected = false;
if (fileName == null) {
return false;
}
@SuppressWarnings("unchecked")
Map<String, String> map = (Map<String, String>) context.getObject(CoreConstants.FA_FILENAME_COLLISION_MAP);
if (map == null) {
return collisionsDetected;
}
for (Entry<String, String> entry : map.entrySet()) {
if (fileName.equals(entry.getValue())) {
addErrorForCollision("File", entry.getValue(), entry.getKey());
collisionsDetected = true;
}
}
if (name != null) {
map.put(getName(), fileName);
}
return collisionsDetected;
}checkForFileCollisionInPreviousFileAppenders方法从上下文读取FA_FILENAME_COLLISION_MAP，判断有没有文件名重复的，有则返回true
openFile
代码语言：javascript复制 public void openFile(String file_name) throws IOException {
lock.lock();
try {
File file = new File(file_name);
boolean result = FileUtil.createMissingParentDirectories(file);
if (!result) {
addError("Failed to create parent directories for [" file.getAbsolutePath() "]");
}
ResilientFileOutputStream resilientFos = new ResilientFileOutputStream(file, append, bufferSize.getSize());
resilientFos.setContext(context);
setOutputStream(resilientFos);
} finally {
lock.unlock();
}
}openFile方法加锁创建file，然后通过createMissingParentDirectories来创建不存在的父目录，最后创建根据file、append参数、bufferSize来创建ResilientFileOutputStream
stop
代码语言：javascript复制 public void stop() {
super.stop();
Map<String, String> map = ContextUtil.getFilenameCollisionMap(context);
if (map == null || getName() == null)
return;
map.remove(getName());
}stop方法会获取FA_FILENAME_COLLISION_MAP，移除当前文件名
writeOut
代码语言：javascript复制 protected void writeOut(E event) throws IOException {
if (prudent) {
safeWrite(event);
} else {
super.writeOut(event);
}
}FileAppender覆盖了OutputStreamAppender的writeOut方法，在prudent为true时执行safeWrite
safeWrite
代码语言：javascript复制 private void safeWrite(E event) throws IOException {
ResilientFileOutputStream resilientFOS = (ResilientFileOutputStream) getOutputStream();
FileChannel fileChannel = resilientFOS.getChannel();
if (fileChannel == null) {
return;
}
// Clear any current interrupt (see LOGBACK-875)
boolean interrupted = Thread.interrupted();
FileLock fileLock = null;
try {
fileLock = fileChannel.lock();
long position = fileChannel.position();
long size = fileChannel.size();
if (size != position) {
fileChannel.position(size);
}
super.writeOut(event);
} catch (IOException e) {
// Mainly to catch FileLockInterruptionExceptions (see LOGBACK-875)
resilientFOS.postIOFailure(e);
} finally {
if (fileLock != null && fileLock.isValid()) {
fileLock.release();
}
// Re-interrupt if we started in an interrupted state (see LOGBACK-875)
if (interrupted) {
Thread.currentThread().interrupt();
}
}
}safeWrite会通过fileChannel.lock()进行加锁，然后执行fileChannel.position(size)，最后通过父类writeOut进行写入；对于IOException会执行resilientFOS.postIOFailure(e)
小结
logback的FileAppender继承了OutputStreamAppender，它定义了append、prudent、bufferSize属性，它使用的是ResilientFileOutputStream，其writeOut方法主要是新增了对prudent模式的支持，在prudent为true时采用的是safeWrite。
