Does Java have a using statement?

问题

Does Java have a using statement that can be used when opening a session in hibernate?

In C# it is something like:

using (var session = new Session())
{


}

So the object goes out of scope and closes automatically.

回答1:

Java 7 introduced Automatic Resource Block Management which brings this feature to the Java platform. Prior versions of Java didn't have anything resembling using.

As an example, you can use any variable implementing java.lang.AutoCloseable in the following way:

try(ClassImplementingAutoCloseable obj = new ClassImplementingAutoCloseable())
{
    ...
}

Java's java.io.Closeable interface, implemented by streams, automagically extends AutoCloseable, so you can already use streams in a try block the same way you would use them in a C# using block. This is equivalent to C#'s using.

As of version 5.0, Hibernate Sessions implement AutoCloseable and can be auto-closed in ARM blocks. In previous versions of Hibernate Session did not implement AutoCloseable. So you'll need to be on Hibernate >= 5.0 in order to use this feature.

回答2:

Before Java 7, there was no such feature in Java (for Java 7 and up see Asaph's answer regarding ARM).

You needed to do it manually and it was a pain:

AwesomeClass hooray = null;
try {
  hooray = new AwesomeClass();
  // Great code
} finally {
  if (hooray!=null) {
    hooray.close();
  }
}

And that's just the code when neither // Great code nor hooray.close() can throw any exceptions.

If you really only want to limit the scope of a variable, then a simple code block does the job:

{
  AwesomeClass hooray = new AwesomeClass();
  // Great code
}

But that's probably not what you meant.

回答3:

Since Java 7 it does: http://blogs.oracle.com/darcy/entry/project_coin_updated_arm_spec

The syntax for the code in the question would be:

try (Session session = new Session())
{
  // do stuff
}

Note that Session needs to implement AutoClosable or one of its (many) sub-interfaces.

回答4:

Technically:

DisposableObject d = null;
try {
    d = new DisposableObject(); 
}
finally {
    if (d != null) {
        d.Dispose();
    }
}

回答5:

The closest java equivalent is

AwesomeClass hooray = new AwesomeClass();
try{
    // Great code
} finally {
    hooray.dispose(); // or .close(), etc.
}

回答6:

No, Java has no using statement equivalent.

回答7:

As of now, no.

However there is a proposal of ARM for Java 7.

回答8:

If you're interested in resource management, Project Lombok offers the @Cleanup annotation. Taken directly from their site:

You can use @Cleanup to ensure a given resource is automatically cleaned up before the code execution path exits your current scope. You do this by annotating any local variable declaration with the @Cleanup annotation like so:

@Cleanup InputStream in = new FileInputStream("some/file");

As a result, at the end of the scope you're in, in.close() is called. This call is guaranteed to run by way of a try/finally construct. Look at the example below to see how this works.

If the type of object you'd like to cleanup does not have a close() method, but some other no-argument method, you can specify the name of this method like so:

@Cleanup("dispose") org.eclipse.swt.widgets.CoolBar bar = new CoolBar(parent, 0);

By default, the cleanup method is presumed to be close(). A cleanup method that takes argument cannot be called via @Cleanup.

Vanilla Java

import java.io.*;

public class CleanupExample {
  public static void main(String[] args) throws IOException {
    InputStream in = new FileInputStream(args[0]);
    try {
      OutputStream out = new FileOutputStream(args[1]);
      try {
        byte[] b = new byte[10000];
        while (true) {
          int r = in.read(b);
          if (r == -1) break;
          out.write(b, 0, r);
        }
      } finally {
        out.close();
      }
    } finally {
      in.close();
    }
  }
}

With Lombok

import lombok.Cleanup;
import java.io.*;

public class CleanupExample {
  public static void main(String[] args) throws IOException {
    @Cleanup InputStream in = new FileInputStream(args[0]);
    @Cleanup OutputStream out = new FileOutputStream(args[1]);
    byte[] b = new byte[10000];
    while (true) {
      int r = in.read(b);
      if (r == -1) break;
      out.write(b, 0, r);
    }
  }
}

回答9:

In java 8 you can use try. Please refer to following page. http://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html

回答10:

Please see this List of Java Keywords.

The using keyword is unfortunately not part of the list.
And there is also no equivalence of the C# using keyword through any other keyword as for now in Java.

To imitate such "using" behaviour, you will have to use a try...catch...finally block, where you would dispose of the resources within finally.

回答11:

ARM blocks, from project coin will be in Java 7. This is feature is intended to bring similar functionality to Java as the .Net using syntax.

回答12:

To answer the question regarding limiting scope of a variable, instead of talking about automatically closing/disposing variables.

In Java you can define closed, anonymous scopes using curly brackets. It's extremely simple.

{
   AwesomeClass hooray = new AwesomeClass()
   // Great code
}

The variable hooray is only available in this scope, and not outside it.

This can be useful if you have repeating variables which are only temporary.

For example, each with index. Just like the item variable is closed over the for loop (i.e., is only available inside it), the index variable is closed over the anonymous scope.

// first loop
{
    Integer index = -1;
    for (Object item : things) {index += 1;
        // ... item, index
    }
}

// second loop
{
    Integer index = -1;
    for (Object item : stuff) {index += 1;
        // ... item, index
    }
}

I also use this sometimes if you don't have a for loop to provide variable scope, but you want to use generic variable names.

{
    User user = new User();
    user.setId(0);
    user.setName("Andy Green");
    user.setEmail("andygreen@gmail.com");
    users.add(user);
}

{
    User user = new User();
    user.setId(1);
    user.setName("Rachel Blue");
    user.setEmail("rachelblue@gmail.com");
    users.add(user);
}

来源：https://stackoverflow.com/questions/2016299/does-java-have-a-using-statement

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