说明
分析的版本是1.2.17,跟最新的log4j2可能有较大出入.下载地址 解压缩后,直接导入成maven工程即可.
代码结构
Log4J核心的类：
- Logger:最核心的类, Most logging operations, except configuration, are done through this class.
- Level:日志级别
- Appender:决定日志输出的目的地
- Layout:日志格式
测试代码
import org.apache.log4j.BasicConfigurator;
import org.apache.log4j.Logger;
public class Test {
public static Logger logger = Logger.getLogger(Test.class);
public static void main(String[] args) throws Exception {
BasicConfigurator.configure();
logger.debug("Hello world.");
logger.info("What a beatiful day.");
}
}
输出:
0 [main] DEBUG log4jtest.Test - Hello world.
0 [main] INFO log4jtest.Test - What a beatiful day.
这是最简单的测试代码,不需要配置文件.
配置文件加载
更多情况下,我们需要使用自己的配置文件,那配置文件又是怎么加载的呢?其实也很简单,在启动的时候org.apache.log4j.LogManager中的static方法会加载log4j的配置文件,如果指定了log4j.configuration,则从指定的文件加载,否则就先读取log4j.xml格式的配置文件,如果没有再读取log4j.properties的文件,如果需要测试此特性,可以在运行的时候加上-Dlog4j.configuration=myLog4jConfig.properties即可.
static {
// By default we use a DefaultRepositorySelector which always returns 'h'.
Hierarchy h = new Hierarchy(new RootLogger((Level) Level.DEBUG));
repositorySelector = new DefaultRepositorySelector(h);
/** Search for the properties file log4j.properties in the CLASSPATH. */
String override = OptionConverter.getSystemProperty(DEFAULT_INIT_OVERRIDE_KEY, null);
// if there is no default init override, then get the resource
// specified by the user or the default config file.
if (override == null || "false".equalsIgnoreCase(override)) {
String configurationOptionStr = OptionConverter.getSystemProperty(DEFAULT_CONFIGURATION_KEY, null);
String configuratorClassName = OptionConverter.getSystemProperty(CONFIGURATOR_CLASS_KEY, null);
URL url = null;
// if the user has not specified the log4j.configuration
// property, we search first for the file "log4j.xml" and then
// "log4j.properties"
if (configurationOptionStr == null) {
url = Loader.getResource(DEFAULT_XML_CONFIGURATION_FILE);
if (url == null) {
url = Loader.getResource(DEFAULT_CONFIGURATION_FILE);
}
} else {
try {
url = new URL(configurationOptionStr);
} catch (MalformedURLException ex) {
// so, resource is not a URL:
// attempt to get the resource from the class path
url = Loader.getResource(configurationOptionStr);
}
}
// If we have a non-null url, then delegate the rest of the
// configuration to the OptionConverter.selectAndConfigure
// method.
if (url != null) {
LogLog.debug("Using URL [" + url + "] for automatic log4j configuration.");
try {
OptionConverter.selectAndConfigure(url, configuratorClassName, LogManager.getLoggerRepository());
} catch (NoClassDefFoundError e) {
LogLog.warn("Error during default initialization", e);
}
} else {
LogLog.debug("Could not find resource: [" + configurationOptionStr + "].");
}
} else {
LogLog.debug("Default initialization of overridden by " + DEFAULT_INIT_OVERRIDE_KEY + "property.");
}
}
不管是从哪个入口进入,最终会调用org.apache.log4j.spi.Configurator.doConfigure()方法.
Spring中的初始化
在很多情况下是结合spring使用的,在web.xml中的配置文件如下:
<context-param>
<param-name>log4jConfigLocation</param-name>
<param-value>classpath:log4j.xml</param-value>
</context-param>
<listener>
<listener-class>org.springframework.web.util.Log4jConfigListener</listener-class>
</listener>
从源代码中可以看出就是调用log4j的PropertyConfigurator.configure方法.
public static void initLogging(String location) throws FileNotFoundException {
String resolvedLocation = SystemPropertyUtils.resolvePlaceholders(location);
URL url = ResourceUtils.getURL(resolvedLocation);
if (ResourceUtils.URL_PROTOCOL_FILE.equals(url.getProtocol()) && !ResourceUtils.getFile(url).exists()) {
throw new FileNotFoundException("Log4j config file [" + resolvedLocation + "] not found");
}
if (resolvedLocation.toLowerCase().endsWith(XML_FILE_EXTENSION)) {
DOMConfigurator.configure(url);
}
else {
PropertyConfigurator.configure(url);
}
}
如何工作
当我们调用logger.info("msg")的时候,到底是怎么发生的呢?
拿最简单的org.apache.log4j.ConsoleAppender举例子
第一步:org.apache.log4j.Category.info(Object)
public void info(Object message) {
if (repository.isDisabled(Level.INFO_INT))
return;
if (Level.INFO.isGreaterOrEqual(this.getEffectiveLevel()))
forcedLog(FQCN, Level.INFO, message, null);
}
第二步:一直会调用父类中的c.aai.appendLoopOnAppenders(event);,除非把c.additive设置成false,默认为true
protected void forcedLog(String fqcn, Priority level, Object message, Throwable t) {
callAppenders(new LoggingEvent(fqcn, this, level, message, t));
}
public void callAppenders(LoggingEvent event) {
int writes = 0;
for (Category c = this; c != null; c = c.parent) {
// Protected against simultaneous call to addAppender, removeAppender,...
synchronized (c) {
if (c.aai != null) {
writes += c.aai.appendLoopOnAppenders(event);
}
if (!c.additive) {
break;
}
}
}
if (writes == 0) {
repository.emitNoAppenderWarning(this);
}
}
第三步:
public int appendLoopOnAppenders(LoggingEvent event) {
int size = 0;
Appender appender;
if (appenderList != null) {
size = appenderList.size();
for (int i = 0; i < size; i++) {
appender = (Appender) appenderList.elementAt(i);
appender.doAppend(event);
}
}
return size;
}
第四步:调用真正的logger的doAppend方法
// Reminder: the nesting of calls is:
//
// doAppend()
// - check threshold
// - filter
// - append();
// - checkEntryConditions();
// - subAppend();
public void append(LoggingEvent event) {
if (!checkEntryConditions()) {
return;
}
subAppend(event);
}
//由于ConsoleAppender没有继承subAppend方法,所以实际就是调用了WriterAppender的subAppend方法
protected void subAppend(LoggingEvent event) {
this.qw.write(this.layout.format(event));
if (layout.ignoresThrowable()) {
String[] s = event.getThrowableStrRep();
if (s != null) {
int len = s.length;
for (int i = 0; i < len; i++) {
this.qw.write(s[i]);
this.qw.write(Layout.LINE_SEP);
}
}
}
if (shouldFlush(event)) {
this.qw.flush();
}
}
//其中的qw由ConsoleAppender初始化
public void activateOptions() {
if (follow) {
if (target.equals(SYSTEM_ERR)) {
setWriter(createWriter(new SystemErrStream()));
} else {
setWriter(createWriter(new SystemOutStream()));
}
} else {
if (target.equals(SYSTEM_ERR)) {
setWriter(createWriter(System.err));
} else {
setWriter(createWriter(System.out));
}
}
super.activateOptions();
}
通过这几个步骤就能把日志打印出来了.