本文接下来分析tomcat的日志记录器，日志记录器是用来记录消息的组件，在tomcat中，日志记录器需要与某个servlet容器相关连；在org.apache.catalina.logger包下，tomcat提供了几种不同类型的日志记录器。
tomcat中的日志记录器都必须实现org.apache.catalina.Logger接口
public interface Logger {
public static final int FATAL = Integer.MIN_VALUE;
public static final int ERROR = 1;
public static final int WARNING = 2;
public static final int INFORMATION = 3;
public static final int DEBUG = 4;
public Container getContainer();
public void setContainer(Container container);
public String getInfo();
public int getVerbosity();
public void setVerbosity(int verbosity);
public void addPropertyChangeListener(PropertyChangeListener listener);
public void log(String message);
public void log(Exception exception, String msg);
public void log(String message, Throwable throwable);
public void log(String message, int verbosity);
public void log(String message, Throwable throwable, int verbosity);
public void removePropertyChangeListener(PropertyChangeListener listener);
}
通常，当传入的日志级别verbosity大于Logger被设置的级别时，message会被记录，否则被忽略
Logger接口共定义了五种日志级别，分别为FATAL ERROR WARNING INFORMATION DEBUG
Tomcat提供了三种类型的日志记录器，分别为SystemOutLogger、SystemErrLogger、FileLogger，在org.apache.catalina.logger包下，均继承自org.apache.catalina.logger.LoggerBase类
LoggerBase类实现了org.apache.catalina.Logger接口，在tomcat5中还实现了Lifecycle接口
LoggerBase类为抽象类，实现了Logger接口中除abstract void log(String msg)外的全部方法实现，所有的其他重载方法最终都会调用该方法（templet模式）,我们还可以调用setVerbosity()方法设置日志级别
具体日志类都比较简单，下面是SystemOutLogger类的实现
public class SystemOutLogger
extends LoggerBase { protected static final String info =
"org.apache.catalina.logger.SystemOutLogger/1.0"; public void log(String msg) { System.out.println(msg); }
}
SystemErrLogger类
public class SystemErrLogger
extends LoggerBase { protected static final String info =
"org.apache.catalina.logger.SystemErrLogger/1.0"; public void log(String msg) { System.err.println(msg);
}
}
FileLogger类稍微复杂一点，将从servlet容器中接收到的消息写入到一个文件中（在tomcat4中，FileLogger类实现了Lifecycle接口）
public class FileLogger
extends LoggerBase
implements Lifecycle {
protected LifecycleSupport lifecycle = new LifecycleSupport(this);
private StringManager sm =
StringManager.getManager(Constants.Package); private boolean started = false; private String suffix = ".log"; private boolean timestamp = false; private PrintWriter writer = null;
public void log(String msg) { // Construct the timestamp we will use, if requested
Timestamp ts = new Timestamp(System.currentTimeMillis());
String tsString = ts.toString().substring(0, 19);
String tsDate = tsString.substring(0, 10); // If the date has changed, switch log files
if (!date.equals(tsDate)) {
synchronized (this) {
if (!date.equals(tsDate)) {
close();
date = tsDate;
open();
}
}
} // Log this message, timestamped if necessary
if (writer != null) {
if (timestamp) {
writer.println(tsString + " " + msg);
} else {
writer.println(msg);
}
} } private void close() { if (writer == null)
return;
writer.flush();
writer.close();
writer = null;
date = "";
} /**
* Open the new log file for the date specified by <code>date</code>.
*/
private void open() { // Create the directory if necessary
File dir = new File(directory);
if (!dir.isAbsolute())
dir = new File(System.getProperty("catalina.base"), directory);
dir.mkdirs(); // Open the current log file
try {
String pathname = dir.getAbsolutePath() + File.separator +
prefix + date + suffix;
writer = new PrintWriter(new FileWriter(pathname, true), true);
} catch (IOException e) {
writer = null;
} } public void addLifecycleListener(LifecycleListener listener) { lifecycle.addLifecycleListener(listener);
}
public LifecycleListener[] findLifecycleListeners() { return lifecycle.findLifecycleListeners();
} public void removeLifecycleListener(LifecycleListener listener) { lifecycle.removeLifecycleListener(listener);
} public void start() throws LifecycleException { // Validate and update our current component state
if (started)
throw new LifecycleException
(sm.getString("fileLogger.alreadyStarted"));
lifecycle.fireLifecycleEvent(START_EVENT, null);
started = true;
} public void stop() throws LifecycleException { // Validate and update our current component state
if (!started)
throw new LifecycleException
(sm.getString("fileLogger.notStarted"));
lifecycle.fireLifecycleEvent(STOP_EVENT, null);
started = false;
close();
} }
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