mybatis-源码解析-基于加载xml(InputStream)

在解析mybatis源码时，通过用例

public static void main(String[] args)throws Exception{
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        List<SysParam> sysParamList =  sqlSession.selectList("com.wolf.sty.mapper.SysParamMapper.selectList","30");
        System.out.println(JSON.toJSONString(sysParamList));
}

本章主要分析
String resource = "mybatis-config.xml";
//如何被加载的
InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new 

mybatis-config.xml配置

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <properties resource="mysql.properties"/>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${driver}"/>
                <property name="url" value="${url}"/>
                <property name="username" value="${username}"/>
                <property name="password" value="${password}"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="mapper/SysParamMapper.xml"/>
        <!--<package name="mapper"/>-->
    </mappers>
</configuration>

开始分析如何加载mybatis-config.xml

String resource = "mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);

在mybatis中通过Resources.getResourceAsStream()进行加载intputStream

Resources.class
//属性
private static ClassLoaderWrapper classLoaderWrapper = new ClassLoaderWrapper();
//
public static InputStream getResourceAsStream(String resource) throws IOException {
  return getResourceAsStream(null, resource);
}
//调用重载方法

public static InputStream getResourceAsStream(ClassLoader loader, String resource) throws IOException {
  //这里调用了classLoaderWrapper的getResourceAsStream
  InputStream in = classLoaderWrapper.getResourceAsStream(resource, loader);
  //如果返回的InputStream为空，即表示给定路径的资源没找到，抛出IO加载异常
  if (in == null) {
    throw new IOException("Could not find resource " + resource);
  }
  return in;
}

ClassLoaderWrapper.class资源加载包装器
public class ClassLoaderWrapper {
  //默认类加载器
  ClassLoader defaultClassLoader;
  //系统加载器
  ClassLoader systemClassLoader;

  ClassLoaderWrapper() {
    try {
      systemClassLoader = ClassLoader.getSystemClassLoader();
    } catch (SecurityException ignored) {
      // AccessControlException on Google App Engine
    }
  }

  /**
   * Get a resource as a URL using the current class path
   *
   * @param resource - the resource to locate
   * @return the resource or null
   */
  public URL getResourceAsURL(String resource) {
    return getResourceAsURL(resource, getClassLoaders(null));
  }

  /**
   * Get a resource from the classpath, starting with a specific class loader
   *
   * @param resource    - the resource to find
   * @param classLoader - the first classloader to try
   * @return the stream or null
   */
  public URL getResourceAsURL(String resource, ClassLoader classLoader) {
    return getResourceAsURL(resource, getClassLoaders(classLoader));
  }

  /**
   * Get a resource from the classpath
   *
   * @param resource - the resource to find
   * @return the stream or null
   */
  public InputStream getResourceAsStream(String resource) {
    return getResourceAsStream(resource, getClassLoaders(null));
  }

  /**
   * Get a resource from the classpath, starting with a specific class loader
   *
   * @param resource    - the resource to find
   * @param classLoader - the first class loader to try
   * @return the stream or null
   */
  public InputStream getResourceAsStream(String resource, ClassLoader classLoader) {
    return getResourceAsStream(resource, getClassLoaders(classLoader));
  }

  /**
   * Find a class on the classpath (or die trying)
   *
   * @param name - the class to look for
   * @return - the class
   * @throws ClassNotFoundException Duh.
   */
  public Class<?> classForName(String name) throws ClassNotFoundException {
    return classForName(name, getClassLoaders(null));
  }

  /**
   * Find a class on the classpath, starting with a specific classloader (or die trying)
   *
   * @param name        - the class to look for
   * @param classLoader - the first classloader to try
   * @return - the class
   * @throws ClassNotFoundException Duh.
   */
  public Class<?> classForName(String name, ClassLoader classLoader) throws ClassNotFoundException {
    return classForName(name, getClassLoaders(classLoader));
  }

  /**
   * Try to get a resource from a group of classloaders
   *
   * @param resource    - the resource to get
   * @param classLoader - the classloaders to examine
   * @return the resource or null
   */
  InputStream getResourceAsStream(String resource, ClassLoader[] classLoader) {
    //ClassLoader包含以下5种
    for (ClassLoader cl : classLoader) {
      if (null != cl) {

        // try to find the resource as passed（尝试通过提供路径进行加载）
        InputStream returnValue = cl.getResourceAsStream(resource);

        // now, some class loaders want this leading "/", so we'll add it and try again if we didn't find the resource（现在，一些类加载器需要这个前导“/”，所以我们将添加它，如果找不到资源，请重试）
        if (null == returnValue) {
          returnValue = cl.getResourceAsStream("/" + resource);
        }

        if (null != returnValue) {
          return returnValue;
        }
      }
    }
    return null;
  }
//
ClassLoader[] getClassLoaders(ClassLoader classLoader) {
    return new ClassLoader[]{
        classLoader,//加载资源时，提供的加载器
        defaultClassLoader,//系统默认的加载器（需通过静态方法设置否则默认为null）
        Thread.currentThread().getContextClassLoader(),//当前线程的类加载器
        getClass().getClassLoader(),//当前Resources的类加载器
        systemClassLoader};//系统默认的类加载器
  }

  /**
   * Get a resource as a URL using the current class path
   *
   * @param resource    - the resource to locate
   * @param classLoader - the class loaders to examine
   * @return the resource or null
   */
  URL getResourceAsURL(String resource, ClassLoader[] classLoader) {

    URL url;

    for (ClassLoader cl : classLoader) {

      if (null != cl) {

        // look for the resource as passed in...
        url = cl.getResource(resource);

        // ...but some class loaders want this leading "/", so we'll add it
        // and try again if we didn't find the resource
        if (null == url) {
          url = cl.getResource("/" + resource);
        }

        // "It's always in the last place I look for it!"
        // ... because only an idiot would keep looking for it after finding it, so stop looking already.
        if (null != url) {
          return url;
        }

      }

    }

    // didn't find it anywhere.
    return null;

  }

  /**
   * Attempt to load a class from a group of classloaders
   *
   * @param name        - the class to load
   * @param classLoader - the group of classloaders to examine
   * @return the class
   * @throws ClassNotFoundException - Remember the wisdom of Judge Smails: Well, the world needs ditch diggers, too.
   */
  Class<?> classForName(String name, ClassLoader[] classLoader) throws ClassNotFoundException {

    for (ClassLoader cl : classLoader) {

      if (null != cl) {

        try {

          return Class.forName(name, true, cl);

        } catch (ClassNotFoundException e) {
          // we'll ignore this until all classloaders fail to locate the class
        }

      }

    }

    throw new ClassNotFoundException("Cannot find class: " + name);

  }

}