java try finally block to close stream

问题:

I want to close my stream in the finally block, but it throws an IOException so it seems like I have to nest another try block in my finally block in order to close the stream. Is that the right way to do it? It seems a bit clunky.

Here's the code:

 public void read() {
    try {
        r = new BufferedReader(new InputStreamReader(address.openStream()));
        String inLine;
        while ((inLine = r.readLine()) != null) {
            System.out.println(inLine);
        }
    } catch (IOException readException) {
        readException.printStackTrace();
    } finally {
        try {
            if (r!=null) r.close();
        } catch (Exception e){
            e.printStackTrace();
        }
    }


}

回答1:

It seems a bit clunky.

It is. At least java7's try with resources fixes that.

Pre java7 you can make a closeStream function that swallows it:

public void closeStream(Closeable s){
    try{
        if(s!=null)s.close();
    }catch(IOException e){
        //Log or rethrow as unchecked (like RuntimException) ;)
    }
}

Or put the try...finally inside the try catch:

try{
    BufferedReader r = new BufferedReader(new InputStreamReader(address.openStream()));
    try{

        String inLine;
        while ((inLine = r.readLine()) != null) {
            System.out.println(inLine);
        }
    }finally{
        r.close();
    }
}catch(IOException e){
    e.printStackTrace();
}

It's more verbose and an exception in the finally will hide one in the try but it's semantically closer to the try-with-resources introduced in Java 7.

回答2:

Also if you're using Java 7, you can use a try-with-resources statement:

try(BufferedReader r = new BufferedReader(new InputStreamReader(address.openStream()))) {
    String inLine;
    while ((inLine = r.readLine()) != null) {
        System.out.println(inLine);
    }
} catch(IOException readException) {
    readException.printStackTrace();
}           

回答3:

In Java 7 you can do this...

try (BufferedReader r = new BufferedReader(...)){
     String inLine;
     while ((inLine = r.readLine()) != null) {
          System.out.println(inLine);
     }
} catch(IOException e) {
   //handle exception
}

• Declaring a variable in the try block requires that it implements AutoCloseable.
• Declaring a variable in the try block also limits its scope to the try block.
• Any variable declared in the try block will automatically have close() called when the try block exits.

It's called a Try with resources statement.

回答4:

Yes it is clunky, ugly and confusing. One possible solution is to use Commons IO which offers a closeQuietly method.

There's a number of questions in the "Related" column on the right hand of this page that are actually duplicates, I advise to look through these for some other ways of dealing with this issue.

回答5:

Like the answer mentioning the Commons IO library, the Google Guava Libraries has a similar helper method for things which are java.io.Closeable. The class is com.google.common.io.Closeables. The function you are looking for is similarly named as Commons IO: closeQuietly().

Or you could roll your own to close a bunch like this: Closeables.close(closeable1, closeable2, closeable3, ...) :

import java.io.Closeable;
import java.util.HashMap;
import java.util.Map;

public class Closeables {
  public Map<Closeable, Exception> close(Closeable... closeables) {

  HashMap<Closeable, Exception> exceptions = null;

  for (Closeable closeable : closeables) {
    try {
      if(closeable != null) closeable.close();
    } catch (Exception e) {
        if (exceptions == null) {
          exceptions = new HashMap<Closeable, Exception>();
        }
        exceptions.put(closeable, e);
      }
    }

    return exceptions;
  }
}

And that even returns a map of any exceptions that were thrown or null if none were.

回答6:

Your approach within finally is correct. If the code that you call in a finally block can possibly throw an exception, make sure that you either handle it, or log it. Never let it bubble out of the finally block.

Within the catch block you are swallowing the exception - which is not correct.

Thanks...

回答7:

public void enumerateBar() throws SQLException {
    Statement statement = null;
    ResultSet resultSet = null;
    Connection connection = getConnection();
    try {
        statement = connection.createStatement();
        resultSet = statement.executeQuery("SELECT * FROM Bar");
        // Use resultSet
    }
    finally {
        try {
            if (resultSet != null)
                resultSet.close();
        }
        finally {
            try {
                if (statement != null)
                    statement.close();
            }
            finally {
                connection.close();
            }
        }
    }
}

private Connection getConnection() {
    return null;
}

source. This sample was useful for me.