在Java项目开发中，日志是必不可少的功能，日志对于快速定位问题，检查日常项目运行状态等有非常重要的作用，但是目前Java日志存在多种框架，如：Slf4j、JUL、JCL、Log4j、Log4j2、Logback等。
框架简介
- Slf4j 日志门面组件
- JCL Commons Logging，简称jcl，Apache基金会项目，日志门面组件
- Log4j Apache基金会项目，日志实现框架
- Log4j 2 是Log4j的升级产品，但是与Log4j不兼容
- Logback 日志实现框架
- JUL java官方的日志实现框架
Java中可用的日志框架有很多，这样就导致一个选择困难问题，到底应该用哪一个框架，如果项目修改日志组件或者升级又该如何做。其实一般都会选择使用外观模式：日志门面组件+桥接器+日志实现框架，这样即使项目更换日志种类，只需更换桥接器和日志实现框架，也就是只更换Jar包就可以了，代码无需做任何改动。
spring-jcl使用得是工厂创建日志实例，通过适配器来自动选择日志实现方式，换句话说就是在类路径放入不同的日志jar包，spring会选择不同得日志实现。
LogFactory类,去除了不推荐的方法
public abstract class LogFactory {
/**
* Convenience method to return a named logger.
* @param clazz containing Class from which a log name will be derived
*/
public static Log getLog(Class<?> clazz) {
return getLog(clazz.getName());
}
/**
* Convenience method to return a named logger.
* @param name logical name of the <code>Log</code> instance to be returned
*/
public static Log getLog(String name) {
return LogAdapter.createLog(name);
}
}
LogAdapter
final class LogAdapter {
private static final String LOG4J_SPI = "org.apache.logging.log4j.spi.ExtendedLogger";
private static final String LOG4J_SLF4J_PROVIDER = "org.apache.logging.slf4j.SLF4JProvider";
private static final String SLF4J_SPI = "org.slf4j.spi.LocationAwareLogger";
private static final String SLF4J_API = "org.slf4j.Logger";
private static final LogApi logApi;
static {
if (isPresent(LOG4J_SPI)) {
if (isPresent(LOG4J_SLF4J_PROVIDER) && isPresent(SLF4J_SPI)) {
// log4j-to-slf4j bridge -> we'll rather go with the SLF4J SPI;
// however, we still prefer Log4j over the plain SLF4J API since
// the latter does not have location awareness support.
logApi = LogApi.SLF4J_LAL;
}
else {
// Use Log4j 2.x directly, including location awareness support
logApi = LogApi.LOG4J;
}
}
else if (isPresent(SLF4J_SPI)) {
// Full SLF4J SPI including location awareness support
logApi = LogApi.SLF4J_LAL;
}
else if (isPresent(SLF4J_API)) {
// Minimal SLF4J API without location awareness support
logApi = LogApi.SLF4J;
}
else {
// java.util.logging as default
logApi = LogApi.JUL;
}
}
private LogAdapter() {
}
/**
* Create an actual {@link Log} instance for the selected API.
* @param name the logger name
*/
public static Log createLog(String name) {
switch (logApi) {
case LOG4J:
return Log4jAdapter.createLog(name);
case SLF4J_LAL:
return Slf4jAdapter.createLocationAwareLog(name);
case SLF4J:
return Slf4jAdapter.createLog(name);
default:
// Defensively use lazy-initializing adapter class here as well since the
// java.logging module is not present by default on JDK 9. We are requiring
// its presence if neither Log4j nor SLF4J is available; however, in the
// case of Log4j or SLF4J, we are trying to prevent early initialization
// of the JavaUtilLog adapter - e.g. by a JVM in debug mode - when eagerly
// trying to parse the bytecode for all the cases of this switch clause.
return JavaUtilAdapter.createLog(name);
}
}
private static boolean isPresent(String className) {
try {
Class.forName(className, false, LogAdapter.class.getClassLoader());
return true;
}
catch (ClassNotFoundException ex) {
return false;
}
}
private enum LogApi {LOG4J, SLF4J_LAL, SLF4J, JUL}
private static class Log4jAdapter {
public static Log createLog(String name) {
return new Log4jLog(name);
}
}
private static class Slf4jAdapter {
public static Log createLocationAwareLog(String name) {
Logger logger = LoggerFactory.getLogger(name);
return (logger instanceof LocationAwareLogger ?
new Slf4jLocationAwareLog((LocationAwareLogger) logger) : new Slf4jLog<>(logger));
}
public static Log createLog(String name) {
return new Slf4jLog<>(LoggerFactory.getLogger(name));
}
}
private static class JavaUtilAdapter {
public static Log createLog(String name) {
return new JavaUtilLog(name);
}
}
@SuppressWarnings("serial")
private static class Log4jLog implements Log, Serializable {
private static final String FQCN = Log4jLog.class.getName();
private static final LoggerContext loggerContext =
LogManager.getContext(Log4jLog.class.getClassLoader(), false);
private final ExtendedLogger logger;
public Log4jLog(String name) {
LoggerContext context = loggerContext;
if (context == null) {
// Circular call in early-init scenario -> static field not initialized yet
context = LogManager.getContext(Log4jLog.class.getClassLoader(), false);
}
this.logger = context.getLogger(name);
}
@Override
public boolean isFatalEnabled() {
return this.logger.isEnabled(Level.FATAL);
}
@Override
public boolean isErrorEnabled() {
return this.logger.isEnabled(Level.ERROR);
}
@Override
public boolean isWarnEnabled() {
return this.logger.isEnabled(Level.WARN);
}
@Override
public boolean isInfoEnabled() {
return this.logger.isEnabled(Level.INFO);
}
@Override
public boolean isDebugEnabled() {
return this.logger.isEnabled(Level.DEBUG);
}
@Override
public boolean isTraceEnabled() {
return this.logger.isEnabled(Level.TRACE);
}
@Override
public void fatal(Object message) {
log(Level.FATAL, message, null);
}
@Override
public void fatal(Object message, Throwable exception) {
log(Level.FATAL, message, exception);
}
@Override
public void error(Object message) {
log(Level.ERROR, message, null);
}
@Override
public void error(Object message, Throwable exception) {
log(Level.ERROR, message, exception);
}
@Override
public void warn(Object message) {
log(Level.WARN, message, null);
}
@Override
public void warn(Object message, Throwable exception) {
log(Level.WARN, message, exception);
}
@Override
public void info(Object message) {
log(Level.INFO, message, null);
}
@Override
public void info(Object message, Throwable exception) {
log(Level.INFO, message, exception);
}
@Override
public void debug(Object message) {
log(Level.DEBUG, message, null);
}
@Override
public void debug(Object message, Throwable exception) {
log(Level.DEBUG, message, exception);
}
@Override
public void trace(Object message) {
log(Level.TRACE, message, null);
}
@Override
public void trace(Object message, Throwable exception) {
log(Level.TRACE, message, exception);
}
private void log(Level level, Object message, Throwable exception) {
if (message instanceof String) {
// Explicitly pass a String argument, avoiding Log4j's argument expansion
// for message objects in case of "{}" sequences (SPR-16226)
if (exception != null) {
this.logger.logIfEnabled(FQCN, level, null, (String) message, exception);
}
else {
this.logger.logIfEnabled(FQCN, level, null, (String) message);
}
}
else {
this.logger.logIfEnabled(FQCN, level, null, message, exception);
}
}
}
@SuppressWarnings("serial")
private static class Slf4jLog<T extends Logger> implements Log, Serializable {
protected final String name;
protected transient T logger;
public Slf4jLog(T logger) {
this.name = logger.getName();
this.logger = logger;
}
@Override
public boolean isFatalEnabled() {
return isErrorEnabled();
}
@Override
public boolean isErrorEnabled() {
return this.logger.isErrorEnabled();
}
@Override
public boolean isWarnEnabled() {
return this.logger.isWarnEnabled();
}
@Override
public boolean isInfoEnabled() {
return this.logger.isInfoEnabled();
}
@Override
public boolean isDebugEnabled() {
return this.logger.isDebugEnabled();
}
@Override
public boolean isTraceEnabled() {
return this.logger.isTraceEnabled();
}
@Override
public void fatal(Object message) {
error(message);
}
@Override
public void fatal(Object message, Throwable exception) {
error(message, exception);
}
@Override
public void error(Object message) {
if (message instanceof String || this.logger.isErrorEnabled()) {
this.logger.error(String.valueOf(message));
}
}
@Override
public void error(Object message, Throwable exception) {
if (message instanceof String || this.logger.isErrorEnabled()) {
this.logger.error(String.valueOf(message), exception);
}
}
@Override
public void warn(Object message) {
if (message instanceof String || this.logger.isWarnEnabled()) {
this.logger.warn(String.valueOf(message));
}
}
@Override
public void warn(Object message, Throwable exception) {
if (message instanceof String || this.logger.isWarnEnabled()) {
this.logger.warn(String.valueOf(message), exception);
}
}
@Override
public void info(Object message) {
if (message instanceof String || this.logger.isInfoEnabled()) {
this.logger.info(String.valueOf(message));
}
}
@Override
public void info(Object message, Throwable exception) {
if (message instanceof String || this.logger.isInfoEnabled()) {
this.logger.info(String.valueOf(message), exception);
}
}
@Override
public void debug(Object message) {
if (message instanceof String || this.logger.isDebugEnabled()) {
this.logger.debug(String.valueOf(message));
}
}
@Override
public void debug(Object message, Throwable exception) {
if (message instanceof String || this.logger.isDebugEnabled()) {
this.logger.debug(String.valueOf(message), exception);
}
}
@Override
public void trace(Object message) {
if (message instanceof String || this.logger.isTraceEnabled()) {
this.logger.trace(String.valueOf(message));
}
}
@Override
public void trace(Object message, Throwable exception) {
if (message instanceof String || this.logger.isTraceEnabled()) {
this.logger.trace(String.valueOf(message), exception);
}
}
protected Object readResolve() {
return Slf4jAdapter.createLog(this.name);
}
}
@SuppressWarnings("serial")
private static class Slf4jLocationAwareLog extends Slf4jLog<LocationAwareLogger> implements Serializable {
private static final String FQCN = Slf4jLocationAwareLog.class.getName();
public Slf4jLocationAwareLog(LocationAwareLogger logger) {
super(logger);
}
@Override
public void fatal(Object message) {
error(message);
}
@Override
public void fatal(Object message, Throwable exception) {
error(message, exception);
}
@Override
public void error(Object message) {
if (message instanceof String || this.logger.isErrorEnabled()) {
this.logger.log(null, FQCN, LocationAwareLogger.ERROR_INT, String.valueOf(message), null, null);
}
}
@Override
public void error(Object message, Throwable exception) {
if (message instanceof String || this.logger.isErrorEnabled()) {
this.logger.log(null, FQCN, LocationAwareLogger.ERROR_INT, String.valueOf(message), null, exception);
}
}
@Override
public void warn(Object message) {
if (message instanceof String || this.logger.isWarnEnabled()) {
this.logger.log(null, FQCN, LocationAwareLogger.WARN_INT, String.valueOf(message), null, null);
}
}
@Override
public void warn(Object message, Throwable exception) {
if (message instanceof String || this.logger.isWarnEnabled()) {
this.logger.log(null, FQCN, LocationAwareLogger.WARN_INT, String.valueOf(message), null, exception);
}
}
@Override
public void info(Object message) {
if (message instanceof String || this.logger.isInfoEnabled()) {
this.logger.log(null, FQCN, LocationAwareLogger.INFO_INT, String.valueOf(message), null, null);
}
}
@Override
public void info(Object message, Throwable exception) {
if (message instanceof String || this.logger.isInfoEnabled()) {
this.logger.log(null, FQCN, LocationAwareLogger.INFO_INT, String.valueOf(message), null, exception);
}
}
@Override
public void debug(Object message) {
if (message instanceof String || this.logger.isDebugEnabled()) {
this.logger.log(null, FQCN, LocationAwareLogger.DEBUG_INT, String.valueOf(message), null, null);
}
}
@Override
public void debug(Object message, Throwable exception) {
if (message instanceof String || this.logger.isDebugEnabled()) {
this.logger.log(null, FQCN, LocationAwareLogger.DEBUG_INT, String.valueOf(message), null, exception);
}
}
@Override
public void trace(Object message) {
if (message instanceof String || this.logger.isTraceEnabled()) {
this.logger.log(null, FQCN, LocationAwareLogger.TRACE_INT, String.valueOf(message), null, null);
}
}
@Override
public void trace(Object message, Throwable exception) {
if (message instanceof String || this.logger.isTraceEnabled()) {
this.logger.log(null, FQCN, LocationAwareLogger.TRACE_INT, String.valueOf(message), null, exception);
}
}
@Override
protected Object readResolve() {
return Slf4jAdapter.createLocationAwareLog(this.name);
}
}
@SuppressWarnings("serial")
private static class JavaUtilLog implements Log, Serializable {
private String name;
private transient java.util.logging.Logger logger;
public JavaUtilLog(String name) {
this.name = name;
this.logger = java.util.logging.Logger.getLogger(name);
}
@Override
public boolean isFatalEnabled() {
return isErrorEnabled();
}
@Override
public boolean isErrorEnabled() {
return this.logger.isLoggable(java.util.logging.Level.SEVERE);
}
@Override
public boolean isWarnEnabled() {
return this.logger.isLoggable(java.util.logging.Level.WARNING);
}
@Override
public boolean isInfoEnabled() {
return this.logger.isLoggable(java.util.logging.Level.INFO);
}
@Override
public boolean isDebugEnabled() {
return this.logger.isLoggable(java.util.logging.Level.FINE);
}
@Override
public boolean isTraceEnabled() {
return this.logger.isLoggable(java.util.logging.Level.FINEST);
}
@Override
public void fatal(Object message) {
error(message);
}
@Override
public void fatal(Object message, Throwable exception) {
error(message, exception);
}
@Override
public void error(Object message) {
log(java.util.logging.Level.SEVERE, message, null);
}
@Override
public void error(Object message, Throwable exception) {
log(java.util.logging.Level.SEVERE, message, exception);
}
@Override
public void warn(Object message) {
log(java.util.logging.Level.WARNING, message, null);
}
@Override
public void warn(Object message, Throwable exception) {
log(java.util.logging.Level.WARNING, message, exception);
}
@Override
public void info(Object message) {
log(java.util.logging.Level.INFO, message, null);
}
@Override
public void info(Object message, Throwable exception) {
log(java.util.logging.Level.INFO, message, exception);
}
@Override
public void debug(Object message) {
log(java.util.logging.Level.FINE, message, null);
}
@Override
public void debug(Object message, Throwable exception) {
log(java.util.logging.Level.FINE, message, exception);
}
@Override
public void trace(Object message) {
log(java.util.logging.Level.FINEST, message, null);
}
@Override
public void trace(Object message, Throwable exception) {
log(java.util.logging.Level.FINEST, message, exception);
}
private void log(java.util.logging.Level level, Object message, Throwable exception) {
if (this.logger.isLoggable(level)) {
LogRecord rec;
if (message instanceof LogRecord) {
rec = (LogRecord) message;
}
else {
rec = new LocationResolvingLogRecord(level, String.valueOf(message));
rec.setLoggerName(this.name);
rec.setResourceBundleName(this.logger.getResourceBundleName());
rec.setResourceBundle(this.logger.getResourceBundle());
rec.setThrown(exception);
}
logger.log(rec);
}
}
protected Object readResolve() {
return new JavaUtilLog(this.name);
}
}
@SuppressWarnings("serial")
private static class LocationResolvingLogRecord extends LogRecord {
private static final String FQCN = JavaUtilLog.class.getName();
private volatile boolean resolved;
public LocationResolvingLogRecord(java.util.logging.Level level, String msg) {
super(level, msg);
}
@Override
public String getSourceClassName() {
if (!this.resolved) {
resolve();
}
return super.getSourceClassName();
}
@Override
public void setSourceClassName(String sourceClassName) {
super.setSourceClassName(sourceClassName);
this.resolved = true;
}
@Override
public String getSourceMethodName() {
if (!this.resolved) {
resolve();
}
return super.getSourceMethodName();
}
@Override
public void setSourceMethodName(String sourceMethodName) {
super.setSourceMethodName(sourceMethodName);
this.resolved = true;
}
private void resolve() {
StackTraceElement[] stack = new Throwable().getStackTrace();
String sourceClassName = null;
String sourceMethodName = null;
boolean found = false;
for (StackTraceElement element : stack) {
String className = element.getClassName();
if (FQCN.equals(className)) {
found = true;
}
else if (found) {
sourceClassName = className;
sourceMethodName = element.getMethodName();
break;
}
}
setSourceClassName(sourceClassName);
setSourceMethodName(sourceMethodName);
}
@SuppressWarnings("deprecation") // setMillis is deprecated in JDK 9
protected Object writeReplace() {
LogRecord serialized = new LogRecord(getLevel(), getMessage());
serialized.setLoggerName(getLoggerName());
serialized.setResourceBundle(getResourceBundle());
serialized.setResourceBundleName(getResourceBundleName());
serialized.setSourceClassName(getSourceClassName());
serialized.setSourceMethodName(getSourceMethodName());
serialized.setSequenceNumber(getSequenceNumber());
serialized.setParameters(getParameters());
serialized.setThreadID(getThreadID());
serialized.setMillis(getMillis());
serialized.setThrown(getThrown());
return serialized;
}
}
}
#######总结
先找log4j再找slf4j，现在主流日志是slf4j+logback组合，slf4j充当门面，logback具体日志实现框架。
回顾我们平时使用日志的方式，就可以兼容各种日志框架，而不需要更改代码
public class LogDemo {
Log log = LogFactory.getLog(LogDemo.class);
public void logLevel() {
log.debug("debug");
log.warn("warn");
log.trace("trace");
log.info("debug");
log.error("debug");
log.fatal("debug");
log.debug("debug");
}
}
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