Try with resources in Java

Apr 23, 2019

Java try with resources

Try with resources offers an easy and foolproof way to make sure all your resources are properly closed. It manages closing automatically instead of explicitly using try-finally.

Traditional try-catch-finally

With traditional try-catch, you can execute a block of code and handle any exceptions which may occur:

try {
    // Do something
} catch (Exception e) {
    // React to exceptions if they occurr
    // in the try block
}

Often it is useful to have also a finally block. It executes after try/catch, no matter whether there was an exception or not. It is very useful, especially when working with resources, which need proper cleanup after being used. When you use some resources, which are locked by your program (such as files or sockets and more), you need to make sure you release them after you're done.

You need to do the cleanup no matter whether all ends well or whether there is an exception.

try {
    // Open a resource
    // Try to use a resource
} catch (Exception e) {
    // React to exceptions if they occur
    // in the try block
} finally {
    // Close the resource when we're done
}

Finally is a nice way to clean everything after we are done, but has its own limitations. There can be multiple places where an exception may occur. That is when opening the resource, using it and even closing. That can result in a lot of boilerplate code, which is hard to read and error prone. We are talking about nested try-catch statements combined with null checks and more.

Most importantly, you need to remember to actually close all the resources and in the right order. That is, resources can have dependencies on each other, which you need to respect.

Fortunately, since Java 7, there is an easier way to manage resources automatically with try-with-resources, which will close all the resources properly for you in the right order.

Try with resources

Using try-with-resources is easy. Instead of just plain:

try {
    // Do something
} finally {
    // Close my resource
}

You create the resource directly after try keyword in parentheses, like this:

try (MyResource resource = new MyResource()) {
    // Do something
} // Resource is automatically closed after this block ends

You basically create a resource, which exists only inside the try block and cannot be accessed afterward. When the try block ends, the resource is automatically closed for you.

Now you ask what kind of resources you can use in try-with-resources and how does JVM know how to close them?

It is actually pretty simple. You can use all the resources, which implement Closeable or AutoCloseable interface. It has only one method called close() which is automatically called for you once the try block finishes. That means it is easy to provide your own custom resources. You can check all the classes implementing Closeable or AutoCloseable in their JavaDoc.

Note that in the example above, we used a simple case of try-with-resources using just try block. Of course, you can include both catch and finally blocks as well. In such case, the catch and finally blocks are executed after all the resource have been closed.

Decompiled example

Let's look at a simple example using try-with-resources, which uses two resources and has just one operation:

try (FileReader fileReader = new FileReader("C:\\foo.txt");
             BufferedReader bufferedReader = new BufferedReader(fileReader)) {

    bufferedReader.readLine();
}

If we decompile this using Fernflower decompiler, we get something similar to this:

FileReader fileReader = new FileReader("C:\\foo.txt");
Throwable var2 = null;

try {
    BufferedReader bufferedReader = new BufferedReader(fileReader);
    Throwable var4 = null;

    try {
        bufferedReader.readLine();
    } catch (Throwable var16) {
        var4 = var16;
        throw var16;
    } finally {
        $closeResource(var4, bufferedReader);
    }
} catch (Throwable var18) {
    var2 = var18;
    throw var18;
} finally {
    $closeResource(var2, fileReader);
}

It is an interesting example of how try-with-resources works under the hood. You can check the official specs for more detail.

Multiple resources

Inside the head of try-with-resources, you can declare more than one resource.

try (FileReader fileReader = new FileReader("C:\\foo.txt");
     BufferedReader bufferedReader = new BufferedReader(fileReader)) {
            // Read some data!
}

All the resources in the () parentheses are automatically closed for you. What's good to know is what is the order of closing. The resources are closed in the reverse order of their declaration to avoid any dependency issues.

That is, if we have three resources like this:

try (resource1; resource2; resource3) {
}

They are created in order of:

  1. resource1
  2. resource2
  3. resource3

However, they are closed in the reverse order:

  1. resource3
  2. resource2
  3. resource1

Java 9 improvements

Try with resources was introduced in Java 7. Until Java 9 you were forced to declare the resources and assign them a value in the parentheses right after try. This is a lot of text and noise, which makes try-with-resources hard to read, especially when using multiple resources.

try (FileReader fileReader = new FileReader("C:\\foo.txt"); 
     BufferedReader bufferedReader = new BufferedReader(fileReader)) {
    // Read some data!
}

Fortunately, since Java 9, you can just reference the name of the existing resource instead of its declaration and assignment. This is much more readable.

FileReader fileReader = new FileReader("C:\\foo.txt");
BufferedReader bufferedReader = new BufferedReader(fileReader);
        
try (fileReader; bufferedReader) {
    // Read some data!
}

It is not shorter, but when you check try-with-resources, you can immediately see which resources it uses without all the noise.

The limitation is, though that all the resources used need to be final of effectively final.

Implementing your own resources

The good news is that you can create your own resources, which can be used in try-with-resources. All you need to do is to implement either java.io.Closeable or java.lang.AutoCloseable.

How are they different? Closeable is older, available since Java 5, AutoCloseable was introduced in Java 7 along with try-with-resources. Since Java 7, Closeable actually extends AutoCloseable.

public interface AutoCloseable {
    void close() throws Exception;
}

public interface Closeable extends AutoCloseable {
    void close() throws IOException;
}

As you can see, they are very similar. The signature of the close() method is different only in the exception thrown. AutoCloseable can throw any Exception while Closeable throws IOException.

Note that when implementing an interface you can change the method signature in a way that it throws a different exception. Or no exception at all. According to the JavaDoc, it is highly recommended to do so in this case. Declare more specific exception in your implementation of close() method or no exception at all if it cannot fail.

There is one more difference, which you cannot see from the method's signature but from JavaDoc only. Closeable is required to be idempotent, AutoCloseable not (although it is highly recommended).

That means you should make sure calling close() multiple times would not cause any trouble.

IntelliJ IDEA integration

As usual, IDEA offers a nice support for try with resources feature. When you are using a resource, which implements AutoCloseable interface, you can surround it with try-with-resources. Just press Alt + Enter to open intention actions popup:

IntelliJ IDEA surround with 'try-with-resources'

Using the same keyboard shortcut, IDEA allows you to convert traditional try-catch-finally to try-with-resources.

IntelliJ IDEA convert to 'try-with-resources'

Alternatively, you can do the reverse operation using the same shortcut - convert try-with-resources to good old try-catch-finally.

Conclusion

Try with resources is a useful alternative to traditional try-catch-finally when working with resources, which need to be properly closed. The resource management is automatically handled for you. You can still use catch and finally blocks, as usual, they get executed after the resources are closed.

If you are on Java 9 and later, you don't need to declare your resources directly in the try header, but you can use previously declared resources, which are final or effectively final.

If you want to use your own resources, you can just implement Closeable or AutoCloseable interface and implement the close() method.


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Vojtech Ruzicka

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