mybatis io 机制

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目录

概 述

在 Mybatis 中，与磁盘文件的交互主要是对 xml 配置文件的读取操作。除了对磁盘的读写，还有对内存中类文件的操作。

ClassLoaderWrapper 和 Resources
源码如下：

public class ClassLoaderWrapper {

  ClassLoader defaultClassLoader;
  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));
  }

测试代码：

  @Test
  void getResourceAsURL() {
    assertNotNull(wrapper.getResourceAsURL(JPETSTORE_PROPERTIES));
  }

  @Test
  void getResourceAsURLNotFound() {
    assertNull(wrapper.getResourceAsURL(RESOURCE_NOT_FOUND));
  }

测试结果：

Resources：
源码如下：

public class Resources {

  private static ClassLoaderWrapper classLoaderWrapper = new ClassLoaderWrapper();

  /**
   * Charset to use when calling getResourceAsReader.
   * null means use the system default.
   */
  private static Charset charset;

  Resources() {
  }

  /**
   * Returns the default classloader (may be null).
   *
   * @return The default classloader
   */
  public static ClassLoader getDefaultClassLoader() {
    return classLoaderWrapper.defaultClassLoader;
  }

  /**
   * Sets the default classloader
   *
   * @param defaultClassLoader - the new default ClassLoader
   */
  public static void setDefaultClassLoader(ClassLoader defaultClassLoader) {
    classLoaderWrapper.defaultClassLoader = defaultClassLoader;
  }

  /**
   * Returns the URL of the resource on the classpath
   *
   * @param resource The resource to find
   * @return The resource
   * @throws java.io.IOException If the resource cannot be found or read
   */
  public static URL getResourceURL(String resource) throws IOException {
    // issue #625
    return getResourceURL(null, resource);
  }

  /**
   * Returns the URL of the resource on the classpath
   *
   * @param loader   The classloader used to fetch the resource
   * @param resource The resource to find
   * @return The resource
   * @throws java.io.IOException If the resource cannot be found or read
   */
  public static URL getResourceURL(ClassLoader loader, String resource) throws IOException {
    URL url = classLoaderWrapper.getResourceAsURL(resource, loader);
    if (url == null) {
      throw new IOException("Could not find resource " + resource);
    }
    return url;
  }

  /**
   * Returns a resource on the classpath as a Stream object
   *
   * @param resource The resource to find
   * @return The resource
   * @throws java.io.IOException If the resource cannot be found or read
   */
  public static InputStream getResourceAsStream(String resource) throws IOException {
    return getResourceAsStream(null, resource);
  }

  /**
   * Returns a resource on the classpath as a Stream object
   *
   * @param loader   The classloader used to fetch the resource
   * @param resource The resource to find
   * @return The resource
   * @throws java.io.IOException If the resource cannot be found or read
   */
  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;
  }

  /**
   * Returns a resource on the classpath as a Properties object
   *
   * @param resource The resource to find
   * @return The resource
   * @throws java.io.IOException If the resource cannot be found or read
   */
  public static Properties getResourceAsProperties(String resource) throws IOException {
    Properties props = new Properties();
    try (InputStream in = getResourceAsStream(resource)) {
      props.load(in);
    }
    return props;
  }

  /**
   * Returns a resource on the classpath as a Properties object
   *
   * @param loader   The classloader used to fetch the resource
   * @param resource The resource to find
   * @return The resource
   * @throws java.io.IOException If the resource cannot be found or read
   */
  public static Properties getResourceAsProperties(ClassLoader loader, String resource) throws IOException {
    Properties props = new Properties();
    try (InputStream in = getResourceAsStream(loader, resource)) {
      props.load(in);
    }
    return props;
  }

测试代码：

 @Test
  void shouldGetUrlForResource() throws Exception {
    URL url = Resources.getResourceURL(JPETSTORE_PROPERTIES);
    assertTrue(url.toString().endsWith("jpetstore/jpetstore-hsqldb.properties"));
  }

ResolveUtil 和 VFS 及其实现类：

static VFS createVFS() {
      // Try the user implementations first, then the built-ins
      List<Class<? extends VFS>> impls = new ArrayList<>();
      impls.addAll(USER_IMPLEMENTATIONS);
      impls.addAll(Arrays.asList((Class<? extends VFS>[]) IMPLEMENTATIONS));

      // Try each implementation class until a valid one is found
      VFS vfs = null;
      for (int i = 0; vfs == null || !vfs.isValid(); i++) {
        Class<? extends VFS> impl = impls.get(i);
        try {
          vfs = impl.getDeclaredConstructor().newInstance();
          if (!vfs.isValid() && log.isDebugEnabled()) {
            log.debug("VFS implementation " + impl.getName()
                + " is not valid in this environment.");
          }
        } catch (InstantiationException | IllegalAccessException | NoSuchMethodException | InvocationTargetException e) {
          log.error("Failed to instantiate " + impl, e);
          return null;
        }
      }

      if (log.isDebugEnabled()) {
        log.debug("Using VFS adapter " + vfs.getClass().getName());
      }

      return vfs;
    }
  }

ResolveUtil 作为一个工具类，主要用于遍历某一个路径下所有的文件，并且找到符合条件的文件，这是两个功能 遍历、筛选。

我们做下测试：

  @Test
  void invoke() throws IOException, NoSuchMethodException {
    VFS vfs = VFS.invoke(VFS.class.getMethod("getInstance"), VFS.class);
    Assertions.assertEquals(vfs, VFS.getInstance());

    Assertions.assertThrows(RuntimeException.class, () -> {
      //java.lang.IllegalArgumentException: wrong number of arguments
      VFS.invoke(VFS.class.getMethod("getInstance"), VFS.class, "unnecessaryArgument");
    });

    Assertions.assertThrows(IOException.class, () -> {
      //InvocationTargetException.getTargetException -> IOException
      VFS.invoke(Resources.class.getMethod("getResourceAsProperties", String.class), Resources.class, "invalidResource");
    });

    Assertions.assertThrows(RuntimeException.class, () -> {
      //Other InvocationTargetException
      VFS.invoke(Integer.class.getMethod("valueOf", String.class), Resources.class, "InvalidIntegerNumber");
    });

  }

小结

参考资料和推荐阅读

1.链接: link.