Mybatis源码——获取InputStream的过程

Mybatis源码——获取InputStream的过程 总览

InputStream is = Resources.getResourceAsStream(“mybatis.xml”);SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(is);session = factory.openSession();mapper = session.getMapper(StudentDao.class);mapper.findAll();session.commit();session.close();

正文

本文只关注第一步，看源码的方式就是按住ctrl一阵点点点。

public static InputStream getResourceAsStream(String resource) throws IOException {
  return getResourceAsStream(null, resource);
}

这里传了个null，“mybatis.xml”

public static InputStream getResourceAsStream(ClassLoader loader, String resource) throws IOException {
  InputStream in = classLoaderWrapper.getResourceAsStream(resource, loader);
  if (in == null) {
    throw new IOException("Could not find resource " + resource);
  }
  return in;
}

这里获取了InputStream，为空则抛异常。
很严谨，资源只有找到和没找到两种情况。

public InputStream getResourceAsStream(String resource, ClassLoader classLoader) {
  return getResourceAsStream(resource, getClassLoaders(classLoader));
}

这里应该获取了类加载器

ClassLoader[] getClassLoaders(ClassLoader classLoader) {
  return new ClassLoader[]{
      classLoader,
      defaultClassLoader,
      Thread.currentThread().getContextClassLoader(),
      getClass().getClassLoader(),
      systemClassLoader};
}

一堆类加载器：

classLoader							传入的null
defaultClassLoader					null
Thread.currentThread().getContextClassLoader()		线程提供的
getClass().getClassLoader()			当前类的
systemClassLoader					ClassLoader.getSystemClassLoader()

最终

InputStream getResourceAsStream(String resource, ClassLoader[] classLoader) {
  for (ClassLoader cl : classLoader) {
    if (null != cl) {

      // try to find the resource as passed
      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
      if (null == returnValue) {
        returnValue = cl.getResourceAsStream("/" + resource);
      }

      if (null != returnValue) {
        return returnValue;
      }
    }
  }
  return null;
}

遍历所有类加载器，找对应资源，找到直接返回。