Exception Handling - Beginning Java 8 Fundamentals - page 367

finally {
// Let us try to close the resource
try {
if (aRes != null) {
aRes.close(); // Close the resource
}
}
catch(Exception e) {
e.printStackTrace();
}
}
With the new try-with-resources construct in Java 7, the above code can be written as
try (AnyResource aRes = create the resource...) {
// Work with the resource here. The resource will be closed automatically.
}
Wow! You were able to write the same logic in just three lines of code using a try-with-resource construct
in Java 7, when it used to take sixteen lines of code. The try-with-resources construct automatically closes the
resources when the program exits the construct. A try-with-resource construct may have one or more catch blocks
and/or a finally block.
On the surface, the try-with-resources construct is as simple as it seems in the above example. However, it
comes with some subtleties that I need to discuss in detail.
You can specify multiple resources in a try-with-resources block. Two resources must be separated by a
semicolon. The last resource must not be followed by a semicolon. The following snippet of code shows some usage of
try-with-resources to use one and multiple resources:
try (AnyResource aRes1 = getResource1()) {
// Use aRes1 here
}
try (AnyResource aRes1 = getResource1();
AnyResource aRes2 = getResource2()) {
// Use aRes1 and aRes2 here
}
The resources that you specify in a try-with-resources are implicitly final . You can declare the resources
final , even though it is redundant to do so.
try (final AnyResource aRes1 = getResource1()) {
// Use aRes1 here
}
A resource that you specify in a try-with-resources must be of the type java.lang.AutoCloseable . Java 7
added the AutoCloseable interface, which has a close() method. When the program exits the try-with-resources
block, the close() method of all the resources is called automatically. In the case of multiple resources, the close()
method is called in the reverse order in which the resources are specified.
Consider a MyResource class as shown in Listing 9-14. It implements the AutoCloseable interface and provides
implementation for the close() method. If the exceptionOnClose instance variable is set to true , its close() method
throws a RuntimeException . Its use() method throws a RuntimeException if the level is zero or less. Let's use the
MyResource class to demonstrate various rules in using the try-with-resources block.