New/strange Java "try()" syntax?

Question

While messing around with the custom formatting options in Eclipse, in one of the sample pieces of code, I saw code as follows:

/**
 * 'try-with-resources'
 */
class Example {
    void foo() {
        try (FileReader reader1 = new FileReader("file1"); FileReader reader2 = new FileReader("file2")) {

        }
    }
}

I've never seen try used like this and I've been coding in Java for 9 years! Does any one know why you would do this? What is a possible use-case / benefit of doing this?

An other pieces of code I saw, I thought was a very useful shorthand so I'm sharing it here as well, it's pretty obvious what it does:

/**
 * 'multi-catch'
 */
class Example {
    void foo() {
        try {
        } catch (IllegalArgumentException | NullPointerException | ClassCastException e) {
            e.printStackTrace();
        }
    }
}

Answer 1

It was added in Java 7. It's called the try-with-resources statement.

/edit

Might as well throw this in here too. You can use the try-with-resources statement to manage Locks if you use a wrapper class like this:

public class CloseableLock implements Closeable {
    private final Lock lock;

    private CloseableLock(Lock l) {
        lock = l;
    }

    public void close() {
        lock.unlock();
    }

    public static CloseableLock lock(Lock l) {
        l.lock();
        return new CloseableLock(l);
    }
}

try(CloseableLock l = CloseableLock.lock(lock)) { // acquire the lock
    // do something
} // release the lock

However, since you have to declare a variable for every resource, the advantage of this is debatable.

Answer 2

This is Java 7's new try-with-resources statement: http://download.oracle.com/javase/7/docs/technotes/guides/language/try-with-resources.html

Answer 3

Those are changes introduced in JDK7.

First statement is a try-with-resources. I don't know exactly why they exist but exceptions are often caused by inputstreams etc, I guess it just improves readability. Edit: thanks to the other answerers, I read the javadoc and I now know that it will close all i/o streams that implement AutoCloseable, omitting the need for a finally block in a lot of situations

Second is a multi-catch, which is really handy when you have different exceptions that you handle in exactly the same way.

Answer 4

Same usage as using(Resource) in C Sharp,which means this resource will be automatic recycled when your program has leaven out of this code block.(Just my opinion)

The try-with-resources statement ensures that each resource is closed at the end of the statement. Any object that implements java.lang.AutoCloseable, which includes all objects which implement java.io.Closeable, can be used as a resource

The try-with-resources Statement

Answer 5

It's called try-with-resource. It's a way so as to not have to clean after yourself as the language will do it for you.

Answer 6

it was added in java 7. It is called try with resources. Try with resources statement feature was introduced in java 7 version. Try with resource statement is a try statement that declares one or more statements. A resource is an object that must be closed after the program is finished with it.

Before java 7 we use finally block to close the resources that we have used in our program. In finally block we have to close all the resources manually that we have used in our program. For more information you can visit try with resources

Answer 7

That is called with a try with resources. in a try with resources, any kind of closable stream declared in the resources section will be closed after the try statement is done. So it pretty much is a

try{
InputStream is;
//Stuff
}finally{
is.close()
}

Answer 8

The try-with-resources statement is a try statement that declares one or more resources. A resource is an object that must be closed after the program is finished with it. The try-with-resources statement ensures that each resource is closed at the end of the statement. Any object that implements java.lang.AutoCloseable, which includes all objects which implement java.io.Closeable, can be used as a resource.