commons-logging 源码之 log4j 集成

￡神魔★判官ぃ 2022-05-22 07:48 135阅读 0赞

**注意**  
需要阅读： [commons-logging 源码分析][commons-logging] 之后才可以阅读本文

*  添加maven引用
 *  基本使用
    
     *  新建测试类
     *  新建 log4j.properties
 *  分析 LogFactoryImpl 的 newInstance 方法
    
     *  discoverLogImplementation
        
         *  createLogFromClass

# 添加maven引用 #

<dependency>
                <groupId>log4j</groupId>
                <artifactId>log4j</artifactId>
                <version>1.2.17</version>
            </dependency>
            <dependency>
                <groupId>commons-logging</groupId>
                <artifactId>commons-logging</artifactId>
                <version>1.2</version>
            </dependency>

# 基本使用 #

## 新建测试类 ##

public class Main {
        public static void main(String[] args) {
            Log logger = LogFactory.getLog(Main.class);
            logger.info("message");
        }
    }

## 新建 log4j.properties ##

log4j.rootLogger=DEBUG, stdout
    
    log4j.appender.stdout=org.apache.log4j.ConsoleAppender
    log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
    log4j.appender.stdout.layout.ConversionPattern=%d %p  %c:%L - %m%n

# 分析 LogFactoryImpl 的 newInstance 方法 #

protected Log newInstance(String name) throws LogConfigurationException {
            Log instance;
            try {
                if (logConstructor == null) {
                    instance = discoverLogImplementation(name);
                }
                else {
                    Object params[] = { name };
                    instance = (Log) logConstructor.newInstance(params);
                }
                if (logMethod != null) {
                    Object params[] = { this };
                    logMethod.invoke(instance, params);
                }
    
                return instance;
    
            } catch (LogConfigurationException lce) {
                throw lce;
    
            } catch (InvocationTargetException e) {
    
                Throwable c = e.getTargetException();
                throw new LogConfigurationException(c == null ? e : c);
            } catch (Throwable t) {
                handleThrowable(t); // may re-throw t
                throw new LogConfigurationException(t);
            }
        }

## discoverLogImplementation ##

private Log discoverLogImplementation(String logCategory)
            throws LogConfigurationException {
            if (isDiagnosticsEnabled()) {
                logDiagnostic("Discovering a Log implementation...");
            }
    
            initConfiguration();
    
            Log result = null;
    
            // See if the user specified the Log implementation to use
            //用户没有指定log类
            String specifiedLogClassName = findUserSpecifiedLogClassName();
    
            if (specifiedLogClassName != null) {
                if (isDiagnosticsEnabled()) {
                    logDiagnostic("Attempting to load user-specified log class '" +
                        specifiedLogClassName + "'...");
                }
    
                result = createLogFromClass(specifiedLogClassName,
                                            logCategory,
                                            true);
                if (result == null) {
                    StringBuffer messageBuffer =  new StringBuffer("User-specified log class '");
                    messageBuffer.append(specifiedLogClassName);
                    messageBuffer.append("' cannot be found or is not useable.");
    
    
                    informUponSimilarName(messageBuffer, specifiedLogClassName, LOGGING_IMPL_LOG4J_LOGGER);
                    informUponSimilarName(messageBuffer, specifiedLogClassName, LOGGING_IMPL_JDK14_LOGGER);
                    informUponSimilarName(messageBuffer, specifiedLogClassName, LOGGING_IMPL_LUMBERJACK_LOGGER);
                    informUponSimilarName(messageBuffer, specifiedLogClassName, LOGGING_IMPL_SIMPLE_LOGGER);
                    throw new LogConfigurationException(messageBuffer.toString());
                }
    
                return result;
            }
    
    
    
            if (isDiagnosticsEnabled()) {
                logDiagnostic(
                    "No user-specified Log implementation; performing discovery" +
                    " using the standard supported logging implementations...");
            }
            // private static final String LOGGING_IMPL_LOG4J_LOGGER = "org.apache.commons.logging.impl.Log4JLogger";
            /* private static final String[] classesToDiscover = { LOGGING_IMPL_LOG4J_LOGGER, "org.apache.commons.logging.impl.Jdk14Logger", "org.apache.commons.logging.impl.Jdk13LumberjackLogger", "org.apache.commons.logging.impl.SimpleLog" }; */
            // 从classesToDiscover创建Log，直到result!=null 或者遍历结束
            for(int i=0; i<classesToDiscover.length && result == null; ++i) {
                result = createLogFromClass(classesToDiscover[i], logCategory, true);
            }
    
            if (result == null) {
                throw new LogConfigurationException
                            ("No suitable Log implementation");
            }
    
            return result;
        }

### createLogFromClass ###

private Log createLogFromClass(String logAdapterClassName,
                                       String logCategory,
                                       boolean affectState)
            throws LogConfigurationException {
    
            if (isDiagnosticsEnabled()) {
                logDiagnostic("Attempting to instantiate '" + logAdapterClassName + "'");
            }
    
            Object[] params = { logCategory };
            Log logAdapter = null;
            Constructor constructor = null;
    
            Class logAdapterClass = null;
            ClassLoader currentCL = getBaseClassLoader();
    
            for(;;) {
                // Loop through the classloader hierarchy trying to find
                // a viable classloader.
                logDiagnostic("Trying to load '" + logAdapterClassName + "' from classloader " + objectId(currentCL));
                try {
                    if (isDiagnosticsEnabled()) {
    
                        URL url;
    
                        String resourceName = logAdapterClassName.replace('.', '/') + ".class";
                        //resourceName = "org/apache/commons/logging/impl/Log4JLogger.class"
                        if (currentCL != null) {
                            url = currentCL.getResource(resourceName );
                        } else {
                            url = ClassLoader.getSystemResource(resourceName + ".class");
                        }
    
                        if (url == null) {
                            logDiagnostic("Class '" + logAdapterClassName + "' [" + resourceName + "] cannot be found.");
                        } else {
                            logDiagnostic("Class '" + logAdapterClassName + "' was found at '" + url + "'");
                        }
                    }
    
                    Class c;
                    try {
                    //尝试载入：org.apache.commons.logging.impl.Log4JLogger ，不存在时就退出
                        c = Class.forName(logAdapterClassName, true, currentCL);
                    } catch (ClassNotFoundException originalClassNotFoundException) {
    
                        String msg = originalClassNotFoundException.getMessage();
                        logDiagnostic("The log adapter '" + logAdapterClassName + "' is not available via classloader " +
                                      objectId(currentCL) + ": " + msg.trim());
                        try {
    
                            c = Class.forName(logAdapterClassName);
                        } catch (ClassNotFoundException secondaryClassNotFoundException) {
                            // no point continuing: this adapter isn't available
                            msg = secondaryClassNotFoundException.getMessage();
                            logDiagnostic("The log adapter '" + logAdapterClassName +
                                          "' is not available via the LogFactoryImpl class classloader: " + msg.trim());
                            break;
                        }
                    }
    // 获取构造器 Constructor(String)
                    constructor = c.getConstructor(logConstructorSignature);
                    // 创建实例，org.apache.commons.logging.impl.Log4JLogger
                    //params=com.aya.Main
                    Object o = constructor.newInstance(params);
    
    
                    if (o instanceof Log) {
                    //日志代理类：org.apache.commons.logging.impl.Log4JLogger
                        logAdapterClass = c;
                        //获的日志代理，退出循环
                        logAdapter = (Log) o;
                        break;
                    }
    
    
                    handleFlawedHierarchy(currentCL, c);
                } catch (NoClassDefFoundError e) {
    
                    String msg = e.getMessage();
                    logDiagnostic("The log adapter '" + logAdapterClassName +
                                  "' is missing dependencies when loaded via classloader " + objectId(currentCL) +
                                  ": " + msg.trim());
                    break;
                } catch (ExceptionInInitializerError e) {
    
                    String msg = e.getMessage();
                    logDiagnostic("The log adapter '" + logAdapterClassName +
                                  "' is unable to initialize itself when loaded via classloader " + objectId(currentCL) +
                                  ": " + msg.trim());
                    break;
                } catch (LogConfigurationException e) {
    
                    throw e;
                } catch (Throwable t) {
                    handleThrowable(t); // may re-throw t
    
                    handleFlawedDiscovery(logAdapterClassName, currentCL, t);
                }
    
                if (currentCL == null) {
                    break;
                }
    
                // try the parent classloader
                // currentCL = currentCL.getParent();
                currentCL = getParentClassLoader(currentCL);
            }
    // 在commons-logging 源码分析已经分析过....
            if (logAdapterClass != null && affectState) {
                // We've succeeded, so set instance fields
                this.logClassName   = logAdapterClassName;
                this.logConstructor = constructor;
    
                // Identify the <code>setLogFactory</code> method (if there is one)
                try {
                    this.logMethod = logAdapterClass.getMethod("setLogFactory", logMethodSignature);
                    logDiagnostic("Found method setLogFactory(LogFactory) in '" + logAdapterClassName + "'");
                } catch (Throwable t) {
                    handleThrowable(t); // may re-throw t
                    this.logMethod = null;
                    logDiagnostic("[INFO] '" + logAdapterClassName + "' from classloader " + objectId(currentCL) +
                                  " does not declare optional method " + "setLogFactory(LogFactory)");
                }
    
                logDiagnostic("Log adapter '" + logAdapterClassName + "' from classloader " +
                              objectId(logAdapterClass.getClassLoader()) + " has been selected for use.");
            }
    
            return logAdapter;
        }

[commons-logging]: https://blog.csdn.net/mz4138/article/details/80733003