Keynote: Java try-with-resources Statement

The try-with-resources statement ensures that each resource is closed
at the end of the statement.
Otherwords, in try-with-resources statement any catch or finally block is run after the resources declared have been closed.

*Resource - is an object that must be closed after the program is finished with it.

Closing a stream when it's no longer needed is very important to avoid serious resource leaks.
Use try-with-resources or close those in finally block to guarantee that both streams will be closed even if an error occurs.

private void readWithout() { 
    // regular approach
    try { 
        FileInputStream input = new FileInputStream("file.dat");
        int c;
        while ((c = in.read()) != -1) {
            System.out.println(byteValue + " ");
        }
    } catch (IOException e) {
        e.printStackTrace();
    } finally {
        if (input != null)
        {
            try {            
                input.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}
private void readWith() { 
    // try-with-resources approach
    try (FileInputStream input = new FileInputStream("file.dat")) {
 int c;
        while ((c = in.read()) != -1) {
            System.out.println(byteValue + " ");
        }
    } catch (IOException) {
        e.printStackTrace();
    }
}

In the try-with-resources statement, the resources are closed in the reverse order of their declaration.

    try (Connection conn = DriverManager.getConnection(url, user, pwd);
          Statement stmt = conn.createStatement()) {
        /*Execute query*/
    } //note: no catch or finally here

So stmt.close() will be called first, followed by conn.close().

If there is an exception thrown from the try block, the exception will be caught by the catch statement as usually, but as we know resources are closed before the catch or finally invocations. So,any exceptions thrown as a result of closing resources at the end of the try block are suppressed if there was also an exception thrown in the try block. These exceptions can be retrieved from the exception thrown by calling the getSuppressed() method on the exception thrown. For example:

    } catch (SQLException se) {
        out.println("SQLException: " + se);
        // Get an array of suppressed exceptions
        Throwable[] suppressed = se.getSuppressed(); 
        for (Throwable t: suppressed) { 
            out.println("Suppressed exception: " + t);
        }

## Try-with-resources *(since JDK7)* * Within `try-with-resources` u can use multiple system resources - all of those will be automatically closed when no longer needed. * Within `try-with-resources` the resources are closed in the reverse order of their declaration. * You can use `try-with-resources` to ensure that a resources are closed regardless of whether the `try` statement completes normally or abruptly. Other words resources will be closed even if some error was occurred into the `try` block. * Each resource should implement `AutoCloseable` *(throws `Exception`)* interface or it's subinterface `Closeable` *(throws `IOException`)*. * It's allowed to use neither `catch` nor `finally` blocks when u're using `try-with-resources`. * Any exception that happened as a result of closing resources at the end of the `try` block are *suppressed* if there was also an exception thrown in the `try` block of `try-with-resources`. ---- ## see Also * [Oracle Tuts - The try-with-resources Statement](https://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html) * [Keynote: Exceptions in Action](http://bloomlab.blogspot.com/2015/01/keynote-exceptions-in-action.html) * [Keynote: Understand Java Collections](http://bloomlab.blogspot.com/2015/05/keynote-understand-java-collections.html) * [Keynote: Threads like trains:)](http://bloomlab.blogspot.com/2015/05/keynote-threads-like-trains.html) * [Keynote: Shadowing (Hiding fields)](http://bloomlab.blogspot.com/2015/01/keynote-shadowing-hiding-fields.html) * [Keynote: Initialization Order flow](http://bloomlab.blogspot.com/2014/12/keynote-initialization-order-flow.html)