从“尝试”中移出资源

Question

我想要创建一个包装资源的类实例。问题:当构造函数抛出时，资源就会丢失。我想找个解决办法。尝试资源是一个看起来很好的构造，但我不能将资源移出它。

例如，包装HTTP客户端的服务客户机：

class ServiceClient implements Closeable {
  ServiceClient(ClosableHTTPClient client) { /* ... */ }
  public close() { client.close() }

  public ServiceClient create(String url) throws IOException {
    try (ClosableHTTPClient client = createHttpClient(url)) {
      return new ServiceClient(client);
    }  // make try-with do not close `client` on success
  }

  public ClosableHTTPClient createHttpClient(String url) {
    return HttpClientBuilder.create()
        .setConnectionManager(createClosableConnectionManager()) // must be closed, when `build` throws 
        .build();
  }
}

Answer 1

--引发异常的构造函数.

class Foo implements AutoClosable {

    private final Bar someResource;

    /**
     * @param someResource whose ownership is taken over.
     */
    Foo(Bar someResource) {
        this.someResource = someResource;
        try {
            ...
        } catch (Throwable e) {
            someResource.close(); // If this.close is not final.
            throw e;
        }
    }

    @Override public void close() { someResource.close(); }
    ...

唯一的解决方案是处理构造函数中抛出的任何异常。有时会忘记标准java类(容量为负值的BufferedReader不会关闭包装好的读取器)。