SPI机制 - Common-Logging

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SPI机制 - Common-Logging
common-logging（也称Jakarta Commons Logging，缩写 JCL）是常用的日志库门面，具体日志库相关可以看这篇。我们看下它是怎么解耦的。

首先，日志实例是通过LogFactory的getLog(String)方法创建的：

public static getLog(Class clazz) throws LogConfigurationException {
    return getFactory().getInstance(clazz);
}

LogFatory是一个抽象类，它负责加载具体的日志实现，分析其Factory getFactory()方法：

public static org.apache.commons.logging.LogFactory getFactory() throws LogConfigurationException {
    // Identify the class loader we will be using
    ClassLoader contextClassLoader = getContextClassLoaderInternal();

    if (contextClassLoader == null) {
        // This is an odd enough situation to report about. This
        // output will be a nuisance on JDK1.1, as the system
        // classloader is null in that environment.
        if (isDiagnosticsEnabled()) {
            logDiagnostic("Context classloader is null.");
        }
    }

    // Return any previously registered factory for this class loader
    org.apache.commons.logging.LogFactory factory = getCachedFactory(contextClassLoader);
    if (factory != null) {
        return factory;
    }

    if (isDiagnosticsEnabled()) {
        logDiagnostic(
                "[LOOKUP] LogFactory implementation requested for the first time for context classloader " +
                        objectId(contextClassLoader));
        logHierarchy("[LOOKUP] ", contextClassLoader);
    }

    // Load properties file.
    //
    // If the properties file exists, then its contents are used as
    // "attributes" on the LogFactory implementation class. One particular
    // property may also control which LogFactory concrete subclass is
    // used, but only if other discovery mechanisms fail..
    //
    // As the properties file (if it exists) will be used one way or
    // another in the end we may as well look for it first.
    // classpath根目录下寻找commons-logging.properties
    Properties props = getConfigurationFile(contextClassLoader, FACTORY_PROPERTIES);

    // Determine whether we will be using the thread context class loader to
    // load logging classes or not by checking the loaded properties file (if any).
    // classpath根目录下commons-logging.properties是否配置use_tccl
    ClassLoader baseClassLoader = contextClassLoader;
    if (props != null) {
        String useTCCLStr = props.getProperty(TCCL_KEY);
        if (useTCCLStr != null) {
            // The Boolean.valueOf(useTCCLStr).booleanValue() formulation
            // is required for Java 1.2 compatibility.
            if (Boolean.valueOf(useTCCLStr).booleanValue() == false) {
                // Don't use current context classloader when locating any
                // LogFactory or Log classes, just use the class that loaded
                // this abstract class. When this class is deployed in a shared
                // classpath of a container, it means webapps cannot deploy their
                // own logging implementations. It also means that it is up to the
                // implementation whether to load library-specific config files
                // from the TCCL or not.
                baseClassLoader = thisClassLoader;
            }
        }
    }

    // 这里真正开始决定使用哪个factory
    // 首先，尝试查找vm系统属性org.apache.commons.logging.LogFactory，其是否指定factory
    // Determine which concrete LogFactory subclass to use.
    // First, try a global system property
    if (isDiagnosticsEnabled()) {
        logDiagnostic("[LOOKUP] Looking for system property [" + FACTORY_PROPERTY +
                "] to define the LogFactory subclass to use...");
    }

    try {
        String factoryClass = getSystemProperty(FACTORY_PROPERTY, null);
        if (factoryClass != null) {
            if (isDiagnosticsEnabled()) {
                logDiagnostic("[LOOKUP] Creating an instance of LogFactory class '" + factoryClass +
                        "' as specified by system property " + FACTORY_PROPERTY);
            }
            factory = newFactory(factoryClass, baseClassLoader, contextClassLoader);
        } else {
            if (isDiagnosticsEnabled()) {
                logDiagnostic("[LOOKUP] No system property [" + FACTORY_PROPERTY + "] defined.");
            }
        }
    } catch (SecurityException e) {
        if (isDiagnosticsEnabled()) {
            logDiagnostic("[LOOKUP] A security exception occurred while trying to create an" +
                    " instance of the custom factory class" + ": [" + trim(e.getMessage()) +
                    "]. Trying alternative implementations...");
        }
        // ignore
    } catch (RuntimeException e) {
        // This is not consistent with the behaviour when a bad LogFactory class is
        // specified in a services file.
        //
        // One possible exception that can occur here is a ClassCastException when
        // the specified class wasn't castable to this LogFactory type.
        if (isDiagnosticsEnabled()) {
            logDiagnostic("[LOOKUP] An exception occurred while trying to create an" +
                    " instance of the custom factory class" + ": [" +
                    trim(e.getMessage()) +
                    "] as specified by a system property.");
        }
        throw e;
    }

    // 第二，尝试使用java spi服务发现机制，载META-INF/services下寻找org.apache.commons.logging.LogFactory实现
    // Second, try to find a service by using the JDK1.3 class
    // discovery mechanism, which involves putting a file with the name
    // of an interface class in the META-INF/services directory, where the
    // contents of the file is a single line specifying a concrete class
    // that implements the desired interface.

    if (factory == null) {
        if (isDiagnosticsEnabled()) {
            logDiagnostic("[LOOKUP] Looking for a resource file of name [" + SERVICE_ID +
                    "] to define the LogFactory subclass to use...");
        }
        try {
            // META-INF/services/org.apache.commons.logging.LogFactory, SERVICE_ID
            final InputStream is = getResourceAsStream(contextClassLoader, SERVICE_ID);

            if (is != null) {
                // This code is needed by EBCDIC and other strange systems.
                // It's a fix for bugs reported in xerces
                BufferedReader rd;
                try {
                    rd = new BufferedReader(new InputStreamReader(is, "UTF-8"));
                } catch (java.io.UnsupportedEncodingException e) {
                    rd = new BufferedReader(new InputStreamReader(is));
                }

                String factoryClassName = rd.readLine();
                rd.close();

                if (factoryClassName != null && !"".equals(factoryClassName)) {
                    if (isDiagnosticsEnabled()) {
                        logDiagnostic("[LOOKUP]  Creating an instance of LogFactory class " +
                                factoryClassName +
                                " as specified by file '" + SERVICE_ID +
                                "' which was present in the path of the context classloader.");
                    }
                    factory = newFactory(factoryClassName, baseClassLoader, contextClassLoader);
                }
            } else {
                // is == null
                if (isDiagnosticsEnabled()) {
                    logDiagnostic("[LOOKUP] No resource file with name '" + SERVICE_ID + "' found.");
                }
            }
        } catch (Exception ex) {
            // note: if the specified LogFactory class wasn't compatible with LogFactory
            // for some reason, a ClassCastException will be caught here, and attempts will
            // continue to find a compatible class.
            if (isDiagnosticsEnabled()) {
                logDiagnostic(
                        "[LOOKUP] A security exception occurred while trying to create an" +
                                " instance of the custom factory class" +
                                ": [" + trim(ex.getMessage()) +
                                "]. Trying alternative implementations...");
            }
            // ignore
        }
    }

    // 第三，尝试从classpath根目录下的commons-logging.properties中查找org.apache.commons.logging.LogFactory属性指定的factory
    // Third try looking into the properties file read earlier (if found)

    if (factory == null) {
        if (props != null) {
            if (isDiagnosticsEnabled()) {
                logDiagnostic(
                        "[LOOKUP] Looking in properties file for entry with key '" + FACTORY_PROPERTY +
                                "' to define the LogFactory subclass to use...");
            }
            String factoryClass = props.getProperty(FACTORY_PROPERTY);
            if (factoryClass != null) {
                if (isDiagnosticsEnabled()) {
                    logDiagnostic(
                            "[LOOKUP] Properties file specifies LogFactory subclass '" + factoryClass + "'");
                }
                factory = newFactory(factoryClass, baseClassLoader, contextClassLoader);

                // TODO: think about whether we need to handle exceptions from newFactory
            } else {
                if (isDiagnosticsEnabled()) {
                    logDiagnostic("[LOOKUP] Properties file has no entry specifying LogFactory subclass.");
                }
            }
        } else {
            if (isDiagnosticsEnabled()) {
                logDiagnostic("[LOOKUP] No properties file available to determine" + " LogFactory subclass from..");
            }
        }
    }

    // 最后，使用后备factory实现，org.apache.commons.logging.impl.LogFactoryImpl
    // Fourth, try the fallback implementation class

    if (factory == null) {
        if (isDiagnosticsEnabled()) {
            logDiagnostic(
                    "[LOOKUP] Loading the default LogFactory implementation '" + FACTORY_DEFAULT +
                            "' via the same classloader that loaded this LogFactory" +
                            " class (ie not looking in the context classloader).");
        }

        // Note: unlike the above code which can try to load custom LogFactory
        // implementations via the TCCL, we don't try to load the default LogFactory
        // implementation via the context classloader because:
        // * that can cause problems (see comments in newFactory method)
        // * no-one should be customising the code of the default class
        // Yes, we do give up the ability for the child to ship a newer
        // version of the LogFactoryImpl class and have it used dynamically
        // by an old LogFactory class in the parent, but that isn't
        // necessarily a good idea anyway.
        factory = newFactory(FACTORY_DEFAULT, thisClassLoader, contextClassLoader);
    }

    if (factory != null) {
        /**
            * Always cache using context class loader.
            */
        cacheFactory(contextClassLoader, factory);

        if (props != null) {
            Enumeration names = props.propertyNames();
            while (names.hasMoreElements()) {
                String name = (String) names.nextElement();
                String value = props.getProperty(name);
                factory.setAttribute(name, value);
            }
        }
    }

    return factory;
}

可以看出，抽象类LogFactory加载具体实现的步骤如下：

从vm系统属性org.apache.commons.logging.LogFactory
使用SPI服务发现机制，发现org.apache.commons.logging.LogFactory的实现
查找classpath根目录commons-logging.properties的org.apache.commons.logging.LogFactory属性是否指定factory实现
使用默认factory实现，org.apache.commons.logging.impl.LogFactoryImpl

LogFactory的getLog()方法返回类型是org.apache.commons.logging.Log接口，提供了从trace到fatal方法。可以确定，如果日志实现提供者只要实现该接口，并且使用继承自org.apache.commons.logging.LogFactory的子类创建Log，必然可以构建一个松耦合的日志系统。
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原文链接：https://pdai.tech/md/java/advanced/java-advanced-spi.html