java - 包装多个 AutoCloseables

Question

tryAutoCloseable-with-resources 很好，但在我看来，在创建包装多个对象的类时，它仍然不足以进行有效的资源管理。例如，考虑

import java.io.*;

class AutocloseableWrapper implements AutoCloseable {
    private FileReader r1;
    private FileReader r2;

    public AutocloseableWrapper(String path1, String path2) throws IOException {
        r1 = new FileReader(path1);
        r2 = new FileReader(path2);
    }

    @Override
    public void close() throws IOException {
        r1.close();
        r2.close();
    }

    public static void main(String[] args) throws IOException {
        try (AutocloseableWrapper w = new AutocloseableWrapper("good-path", "bad-path")) {
                System.out.format("doing something\n");
                throw new IOException("doing something in main");
            }
    }
}

这个包装器至少有两个问题：

1. 如果“bad-path”无效并导致分配r2抛出，r1则不关闭。
2. 如果包装器构造成功但随后r1.close抛出，r2则不关闭。

所有这些问题都可以解决，但是编写包装器变得非常重要且容易出错，即使只包装两个资源：

import java.io.*;

class AutocloseableWrapper implements AutoCloseable {
    private FileReader r1;
    private FileReader r2;

    public AutocloseableWrapper(String path1, String path2) throws IOException {
        r1 = new FileReader(path1);
        try {
            r2 = new FileReader(path2);
        }
        catch (IOException e) {
            try {
                r1.close();
            }
            catch (IOException e2) {
                e.addSuppressed(e2);
            }
            throw e;
        }
    }

    @Override
    public void close() throws IOException {
        IOException e = null;
        try {
            r1.close();
        }
        catch (IOException e1) {
            e = e1;
        }

        try {
            r2.close();
        }
        catch (IOException e2) {
            if (e == null)
                throw e2;
            else {
                e.addSuppressed(e2);
                throw e;
            }
        }
    }

    public static void main(String[] args) throws IOException {
        try (AutocloseableWrapper w = new AutocloseableWrapper("good-path", "bad-path")) {
                System.out.format("doing something\n");
                throw new IOException("doing something in main");
            }
    }
}

是否有一些帮助类或任何其他方法可以使编写包装器更容易？

Answer 1

您应该启用编译器解包的语法代码....您可以在这里找到 Oracle 文章：- http://www.oracle.com/technetwork/articles/java/trywithresources-401775.html

来到这个问题，如果你有一个包装，你可以做这样的事情

@Override
public void close() throws IOException {
    Throwable t = null;
    try {
        r1.close();
    } catch (Throwable t1) {
        t = t1;
        throw t1;
    } finally {
        if (t != null) {
            try {
                r2.close();
            } catch (Throwable t2) {
                t.addSuppressed(t2);
            }
        } else {

            r2.close();
        }
    }
}

注意：由于 Java 7中的精确重新抛出功能，这将起作用

Answer 2

您可以使用通用资源包装器，例如：

public class CloseableChain implements AutoCloseable {
  private AutoCloseable r1;
  private CloseableChain r2;

  public void attach(AutoCloseable r) {
    if (r1 == null) {
      r1 = r;
    } else {
      if (r2 == null) {
        r2 = new CloseableChain();
      }
      r2.attach(r);
    }
  }

  public void close() throws Exception {
    if (r1 == null) {
      return;
    }
    Throwable t = null;
    try {
      r1.close();
    } catch (Throwable t1) {
      t = t1;
      throw t1;
    } finally {
      if (r2 != null) {
        if (t != null) {
          try {
            r2.close();
          } catch (Throwable t2) {
            t.addSuppressed(t2);
          }
        } else {
          r2.close();
        }
}}}}

然后你可以重构你的代码：

import java.io.*;

class AutocloseableWrapper implements AutoCloseable {
  private CloseableChain chain;
  private FileReader r1;
  private FileReader r2;
  private FileReader r3;

  public AutocloseableWrapper(String path1, String path2) throws IOException {
    chain = new CloseableChain();
    r1 = new FileReader(path1);
    chain.attach(r1);
    r2 = new FileReader(path2);
    chain.attach(r2);
    // and even more...
    r3 = new FileReader("whatever");
    chain.attach(r3);
  }

  @Override
  public void close() throws IOException {
    chain.close();
  }

  public static void main(String[] args) throws IOException {
    try (AutocloseableWrapper w = new AutocloseableWrapper("good", "bad")) {
      System.out.format("doing something\n");
      throw new IOException("doing something in main");
    }
  }
}