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以下为自己刚学习java遇到及排错过程

日志换成了 slf4j+logback,没看到本地有日志文件生成，调试时发现有错误提示(是在加载druid时出现的):

SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.

　网上说没有配置实际的日志实现包导致，但在pom.xml中是有配置的:

    <!-- https://mvnrepository.com/artifact/org.slf4j/slf4j-api -->
    <dependency>
      <groupId>org.slf4j</groupId>
      <artifactId>slf4j-api</artifactId>
      <version>1.7.26</version>
    </dependency>

    <dependency>
      <groupId>ch.qos.logback</groupId>
      <artifactId>logback-core</artifactId>
      <version>1.2.3</version>
    </dependency>
    <!-- https://mvnrepository.com/artifact/ch.qos.logback/logback-classic -->
    <dependency>
      <groupId>ch.qos.logback</groupId>
      <artifactId>logback-classic</artifactId>
      <version>1.2.3</version>
      <scope>test</scope>  // 注意此处
    </dependency>

　　找到出错的源码:com\alibaba\druid\1.1.16\druid-1.1.16-sources.jar!\com\alibaba\druid\pool\DruidAbstractDataSource.java

public abstract class DruidAbstractDataSource extends WrapperAdapter 
implements DruidAbstractDataSourceMBean, DataSource, DataSourceProxy, Serializable 
{
    private static final long serialVersionUID = 1L;
    private final static Log  LOG  = LogFactory.getLog(DruidAbstractDataSource.class); // 此处
	...
}

　　com\alibaba\druid\1.1.16\druid-1.1.16-sources.jar!\com\alibaba\druid\support\logging\LogFactory.java

public class LogFactory {

    private static Constructor logConstructor;

    static {
        String logType= System.getProperty("druid.logType");
        if(logType != null){
            if(logType.equalsIgnoreCase("slf4j")){
                tryImplementation("org.slf4j.Logger", "com.alibaba.druid.support.logging.SLF4JImpl");
            }else if(logType.equalsIgnoreCase("log4j")){
                tryImplementation("org.apache.log4j.Logger", "com.alibaba.druid.support.logging.Log4jImpl");
            }else if(logType.equalsIgnoreCase("log4j2")){
                tryImplementation("org.apache.logging.log4j.Logger", "com.alibaba.druid.support.logging.Log4j2Impl");
            }else if(logType.equalsIgnoreCase("commonsLog")){
                tryImplementation("org.apache.commons.logging.LogFactory",
                        "com.alibaba.druid.support.logging.JakartaCommonsLoggingImpl");
            }else if(logType.equalsIgnoreCase("jdkLog")){
                tryImplementation("java.util.logging.Logger", "com.alibaba.druid.support.logging.Jdk14LoggingImpl");
            }
        }
        // 优先选择log4j,而非Apache Common Logging. 因为后者无法设置真实Log调用者的信息
        tryImplementation("org.slf4j.Logger", "com.alibaba.druid.support.logging.SLF4JImpl"); // 此处
        tryImplementation("org.apache.log4j.Logger", "com.alibaba.druid.support.logging.Log4jImpl");
        tryImplementation("org.apache.logging.log4j.Logger", "com.alibaba.druid.support.logging.Log4j2Impl");
        tryImplementation("org.apache.commons.logging.LogFactory",
                          "com.alibaba.druid.support.logging.JakartaCommonsLoggingImpl");
        tryImplementation("java.util.logging.Logger", "com.alibaba.druid.support.logging.Jdk14LoggingImpl");

        if (logConstructor == null) {
            try {
                logConstructor = NoLoggingImpl.class.getConstructor(String.class);
            } catch (Exception e) {
                throw new IllegalStateException(e.getMessage(), e);
            }
        }
    }
}

private static void tryImplementation(String testClassName, String implClassName) {
	if (logConstructor != null) {
		return;
	}

	try {
		Resources.classForName(testClassName);
		Class implClass = Resources.classForName(implClassName);
		logConstructor = implClass.getConstructor(new Class[] { String.class });

		Class<?> declareClass = logConstructor.getDeclaringClass();
		if (!Log.class.isAssignableFrom(declareClass)) {
			logConstructor = null;
		}

		try {
			if (null != logConstructor) {
				logConstructor.newInstance(LogFactory.class.getName()); // 此处
			}
		} catch (Throwable t) {
			logConstructor = null;
		}

	} catch (Throwable t) {
		// skip
	}
}

com\alibaba\druid\1.1.16\druid-1.1.16-sources.jar!\com\alibaba\druid\support\logging\SLF4JImpl.java

public class SLF4JImpl implements Log {

    private static final String callerFQCN = SLF4JImpl.class.getName();
    private static final Logger testLogger = LoggerFactory.getLogger(SLF4JImpl.class); // 此处
    static {
        // if the logger is not a LocationAwareLogger instance, it can not get correct stack StackTraceElement
        // so ignore this implementation.
        if (!(testLogger instanceof LocationAwareLogger)) {
            throw new UnsupportedOperationException(testLogger.getClass() + " is not a suitable logger");
        }
    }
	...
}

org\slf4j\slf4j-api\1.7.26\slf4j-api-1.7.26-sources.jar!\org\slf4j\LoggerFactory.java

    public static Logger getLogger(Class<?> clazz) {
        Logger logger = getLogger(clazz.getName()); // 此处
        if (DETECT_LOGGER_NAME_MISMATCH) {
            Class<?> autoComputedCallingClass = Util.getCallingClass();
            if (autoComputedCallingClass != null && nonMatchingClasses(clazz, autoComputedCallingClass)) {
                Util.report(String.format("Detected logger name mismatch. Given name: \"%s\"; computed name: \"%s\".", logger.getName(),
                                autoComputedCallingClass.getName()));
                Util.report("See " + LOGGER_NAME_MISMATCH_URL + " for an explanation");
            }
        }
        return logger;
    }

    public static Logger getLogger(String name) {
        ILoggerFactory iLoggerFactory = getILoggerFactory(); // 此处
        return iLoggerFactory.getLogger(name);
    }

public static ILoggerFactory getILoggerFactory() {
        if (INITIALIZATION_STATE == UNINITIALIZED) {
            synchronized (LoggerFactory.class) {
                if (INITIALIZATION_STATE == UNINITIALIZED) {
                    INITIALIZATION_STATE = ONGOING_INITIALIZATION;
                    performInitialization(); // 此处
                }
            }
        }
        switch (INITIALIZATION_STATE) {
        case SUCCESSFUL_INITIALIZATION:
            return StaticLoggerBinder.getSingleton().getLoggerFactory();
        case NOP_FALLBACK_INITIALIZATION:
            return NOP_FALLBACK_FACTORY;
        case FAILED_INITIALIZATION:
            throw new IllegalStateException(UNSUCCESSFUL_INIT_MSG);
        case ONGOING_INITIALIZATION:
            // support re-entrant behavior.
            // See also http://jira.qos.ch/browse/SLF4J-97
            return SUBST_FACTORY;
        }
        throw new IllegalStateException("Unreachable code");
    }

    private final static void performInitialization() {
        bind(); // 此处
        if (INITIALIZATION_STATE == SUCCESSFUL_INITIALIZATION) {
            versionSanityCheck();
        }
    }

private final static void bind() {
        try {
            ...
            // skip check under android, see also
            // http://jira.qos.ch/browse/SLF4J-328
            if (!isAndroid()) {
                staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet(); // 此处
                reportMultipleBindingAmbiguity(staticLoggerBinderPathSet);
            }
            ...
        } catch () {
			...
        } 
    }

static Set<URL> findPossibleStaticLoggerBinderPathSet() {
        // use Set instead of list in order to deal with bug #138
        // LinkedHashSet appropriate here because it preserves insertion order
        // during iteration
        Set<URL> staticLoggerBinderPathSet = new LinkedHashSet<URL>();
        try {
            ClassLoader loggerFactoryClassLoader = LoggerFactory.class.getClassLoader();
            Enumeration<URL> paths;
            if (loggerFactoryClassLoader == null) {
                paths = ClassLoader.getSystemResources(STATIC_LOGGER_BINDER_PATH);
            } else {
                paths = loggerFactoryClassLoader.getResources(STATIC_LOGGER_BINDER_PATH);
            }
            while (paths.hasMoreElements()) {
                URL path = paths.nextElement();
                staticLoggerBinderPathSet.add(path);
            }
        } catch (IOException ioe) {
            ...
        }

		return staticLoggerBinderPathSet; // 返回的size为零
    }

　　找到生成目录，缺少 logback-classic-1.2.3.jar

 最终发现是因为pom.xml中影响(这段代码直接从maven复制)

最终是此句影响，还是基础不劳呀