java-try-with-resources
Java – Try with Resources
1. Overview
2. Using try-with-resources
Simply put, to be auto-closed, a resource must be both declared and initialized inside the try, as shown below:
try (PrintWriter writer = new PrintWriter(new File("test.txt"))) {
writer.println("Hello World");
}3. Replacing try–catch-finally with try-with-resources
The simple and obvious way to use the new try-with-resources functionality is to replace the traditional and verbose try-catch-finally block.
Let’s compare the following code samples – first is a typical try-catch-finally block, then the new approach, using an equivalent try-with-resources block:
Scanner scanner = null;
try {
scanner = new Scanner(new File("test.txt"));
while (scanner.hasNext()) {
System.out.println(scanner.nextLine());
}
} catch (FileNotFoundException e) {
e.printStackTrace();
} finally {
if (scanner != null) {
scanner.close();
}
}And here’s the super succinct solution using try-with-resources:
try (Scanner scanner = new Scanner(new File("test.txt"))) {
while (scanner.hasNext()) {
System.out.println(scanner.nextLine());
}
} catch (FileNotFoundException fnfe) {
fnfe.printStackTrace();
}4. try-with-resources with Multiple Resources
Multiple resources can be declared just fine in a try-with-resources block by separating them with semicolon:
try (Scanner scanner = new Scanner(new File("testRead.txt"));
PrintWriter writer = new PrintWriter(new File("testWrite.txt"))) {
while (scanner.hasNext()) {
writer.print(scanner.nextLine());
}
}5. A Custom Resource with AutoCloseable
To construct a custom resource that will be correctly handled by a try-with-resources block, the class should implement the Closeable or AutoCloseable interfaces, and override the close method:
public class MyResource implements AutoCloseable {
@Override
public void close() throws Exception {
System.out.println("Closed MyResource");
}
}6. Resource Closing Order
Resources that were defined/acquired first will be closed last; let’s look at an example of this behavior:
Resource 1:
public class AutoCloseableResourcesFirst implements AutoCloseable {
public AutoCloseableResourcesFirst() {
System.out.println("Constructor -> AutoCloseableResources_First");
}
public void doSomething() {
System.out.println("Something -> AutoCloseableResources_First");
}
@Override
public void close() throws Exception {
System.out.println("Closed AutoCloseableResources_First");
}
}
Resource 2:
public class AutoCloseableResourcesSecond implements AutoCloseable {
public AutoCloseableResourcesSecond() {
System.out.println("Constructor -> AutoCloseableResources_Second");
}
public void doSomething() {
System.out.println("Something -> AutoCloseableResources_Second");
}
@Override
public void close() throws Exception {
System.out.println("Closed AutoCloseableResources_Second");
}
}
Code:
private void orderOfClosingResources() throws Exception {
try (AutoCloseableResourcesFirst af = new AutoCloseableResourcesFirst();
AutoCloseableResourcesSecond as = new AutoCloseableResourcesSecond()) {
af.doSomething();
as.doSomething();
}
}
7. catch & finally
8. Conclusion
In this article, we have discussed how to use try-with-resources, how to replace try, catch and finally with try-with-resources, building custom resources with AutoCloseable and order in which resources are closed.
The complete source code for the example is available in this GitHub project.
