The try-with-resources Statement

It is always required to close the resources like database connections, file handles like BufferedReader,BufferedWriter etc after it's use.Otherwise we will face resource leak issue.And sometimes we forget to close the resources after the use.But in Java 7 a functional interface namely AutoCloseable is introduced.And resoureces like Connection ,BufferedReader and BufferedWriter etc extends AutoCloseable.
The try-with-resources statement is a try statement that declares one or more resources. The try-with-resources statement ensures that each resource is closed at the end of the try statement. Any object that implements java.lang.AutoCloseable,  can be used as a resource inside the try-with-resources statement.
The following example writes a line in a file. It uses an instance of BufferedWriter to write data in the file. BufferedWriter is a resource that must be closed after the program is finished with it:

static String writeALineToFile(String path) throws IOException {
    try (BufferedWriter bw =
                   new BufferedWriter(new FileWriter(new File("path")))) {
        return bw.write();
    }
}

In this example, the resource declared in the try-with-resources statement is a BufferedWriter. The declaration statement appears within parentheses immediately after the try keyword. The class BufferedWriter, in Java SE 7 and later, implements the interface java.lang.AutoCloseable. Because the BufferedWriter instance is declared in a try-with-resource statement, it will be closed regardless of whether the try statement completes normally or abruptly (as a result of the method BufferedWriter.write throwing an IOException).

Let's make it clear that here try-with-resource statement and try block are two different things.

Prior to Java SE 7, we can use a finally block to ensure that a resource is closed regardless of whether the try statement completes normally or abruptly. The following example uses a finally block instead of a try-with-resources statement:

static String writeALineToFileWithFinally(String path)
                                                     throws IOException {
 try {
       BufferedWriter bw =
                   new BufferedWriter(new FileWriter(new File("path")))) ;
         bw.write();
    }
}
    } finally {
        if (bw != null) bw.close();
    }
}

However, in this example, if the methods write and close both throw exceptions, then the method  
writeALineToFileWithFinally   throws the exception thrown from the finally block; the exception thrown from the try block is suppressed.

But In contrast, in the example readFirstLineFromFile, if exceptions are thrown from both the try block and the try-with-resources statement, then the method readFirstLineFromFile throws the exception thrown from the try block; the exception thrown from the try-with-resources block is suppressed.

We  can retrieve the suppressed exceptions by calling the Throwable.getSuppressed method from the exception thrown by the try block.


Note: A try-with-resources statement can have catch and finally blocks just like an ordinary try statement. In a try-with-resources statement, any catch or finally block is run after the resources declared have been closed.