使用try-with-resources时，当某个自定义类实现可关闭接口时，FileInputStream实例是如何自动创建的

Question

try(FileReader rd = new FileReader("Test.txt");){} 
catch (Exception e) {e.printStackTrace();}

这就是我的研究过程开始的地方。

在做了一些调试之后，我发现FileReader扩展了InputStreamReader。在FileReader类中，此构造函数调用

public FileReader(File file) throws FileNotFoundException {
  super(new FileInputStream(file));
}

它创建FileInputStream类的对象。FileInputStream扩展了InputStream，后者实现了可关闭的接口。

public void close() throws IOException {
        synchronized (closeLock) {
            if (closed) {
                return;
            }
            closed = true;
        }
        if (channel != null) {
           channel.close();
        }

        fd.closeAll(new Closeable() {
            public void close() throws IOException {
               close0();
           }
        });
    }

public class MyClass implements Closeable 
    {
        public void close() 
       {
         System.out.println("connection closed...");
       }
    }

try(MyClass rd = new MyClass();)
{} 
catch (Exception e) 
{e.printStackTrace();}

public void close() throws IOException {
        synchronized (closeLock) {
            if (closed) {
                return;
            }
            closed = true;
        }
        if (channel != null) {
           channel.close();
        }

        fd.closeAll(new Closeable() {
            public void close() throws IOException {
               close0();
           }
        });
    }

提前谢了。

Answer 1

在回答实际问题之前，了解资源尝试是如何工作的是很重要的。为此，我们可以参考Java语言规范(JLS)的§14.20.3。从本质上讲，这一章告诉您的是，如果您有以下代码：

try (AutoCloseable closeable = ...) {
  // try something
}

^{注意:try-with-resources语句适用于任何java.lang.autocloseable实现。java.io.closeable接口扩展了java.lang.autocloseable接口。}

然后由编译器翻译，就像您写了类似的内容：

AutoCloseable closeable = ...;
Throwable primary = null;
try {
  // try something
} catch (Throwable t) {
  primary = t;
  throw t;
} finally {
  if (closeable != null) {
    if (primary != null) {
      try {
        closeable.close();
      catch (Throwable suppressed) {
        primary.addSuppressed(suppressed);
      }
    } else {
      closeable.close();
    }
  }
}

try (AutoCloseable closeable = ...) {
  // try something
} catch (Exception ex) {
  // handle error
} finally {
  // finally...
}

try {
  // "basic" translation
} catch (Exception ex) {
  // handle error
} finally {
  // finally...
}

import java.io.Closeable;
import java.io.IOException;

public class Main {

  public static void main(String[] args) {
    try (MyClass foo = new MyClass()) {
      // try something...
    } catch (IOException ex) {
      ex.printStackTrace();
    }
  }

  public static class MyClass implements Closeable {
    
    @Override
    public void close() throws IOException {
      System.out.println("MyClass#close()");
    }
  }
}

import java.io.Closeable;
import java.io.IOException;

public class Main {

  public static void main(String[] args) {
    MyClass foo = new MyClass();
    Throwable primary = null;
    try {
      try {
        // try something...
      } catch (Throwable t) {
        primary = t;
        throw t;
      } finally {
        if (foo != null) {
          if (primary != null) {
            try {
              foo.close();
            } catch (Throwable suppressed) {
              primary.addSuppressed(suppressed);
            }
          } else {
            foo.close();
          }
        }
      }
    } catch (IOException ex) {
      ex.printStackTrace();
    }
  }

  public static class MyClass implements Closeable {

    @Override
    public void close() throws IOException {
      System.out.println("MyClass#close()");
    }
  }
}