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try-with-resources

 
meeta gaur
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Use try-with-resources
The try-with-resources statement is a try statement that declares one or more resources.

A resource is as an object that must be closed after the program is finished with it. The try-with-resources statement ensures that each resource is closed at the end of the statement. Any object that implements java.lang.AutoCloseable, which includes all objects which implement java.io.Closeable, can be used as a resource:

package java.lang;

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


The Closeable interface extends the AutoCloseable interface. The close() method of the Closeable interface throws exceptions of type IOException while the close() method of the AutoCloseable interface throws exceptions of type Exception. Consequently, subclasses of the AutoCloseable interface can override this behavior of the close() method to throw specialized exceptions, such as IOException, or no exception at all.

package java.io;

import java.io.IOException;

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


The following example reads the first line from a file. It uses an instance of BufferedReader to read data from the file. BufferedReader is a resource that must be closed after the program is finished with it:

static String readFirstLineFromFile(String path) throws IOException {
try (BufferedReader br = new BufferedReader(new FileReader(path))) {
return br.readLine();
}
}


In this example, the resource declared in the try-with-resources statement is a BufferedReader. The declaration statement appears within parentheses immediately after the try keyword. The class BufferedReader, in Java SE 7 and later, implements the interface java.lang.AutoCloseable. Because the BufferedReader 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 BufferedReader.readLine throwing an IOException).

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

static String readFirstLineFromFileWithFinallyBlock(String path) throws IOException {
BufferedReader br = new BufferedReader(new FileReader(path));
try {
return br.readLine();
} finally {
if (br != null) br.close();
}
}


However, in this example, if the methods readLine and close both throw exceptions, then the method readFirstLineFromFileWithFinallyBlock throws the exception thrown from the finally block; the exception thrown from the try block is suppressed. 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. In Java SE 7 and later, you can retrieve suppressed exceptions; see the next section for more information.

You may declare one or more resources in a try-with-resources statement. The following example makes a copy of a file, using the try-with-resources statement. There are two resources defined in the try statement, separated by a semicolon, which are automatically closed when the statement completes:

public static void copyFile(String src, String dest) throws IOException {
try (BufferedReader in = new BufferedReader(new FileReader(src));
BufferedWriter out = new BufferedWriter(new FileWriter(dest))) {
String line;
while((line = in.readLine()) != null) {
out.write(line);
out.write('\n');
}
} // No need to close resources in a "finally"
}


NOTE: the close() methods of resources are called in the OPPOSITE order of their creation.

NOTE: in try-with-resource statement the catch and the finally blocks are OPTIONAL.


I couldn't figure out meaning of NOTE: the close() methods of resources are called in the OPPOSITE order of their creation.
 
Steve Luke
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What's the source of that quote?
 
meeta gaur
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http://www.javacertifications.net/javacert/Language%20Enhancements.jsp
 
Stuie Clarky
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If it closes in the opposite order, then the resource specified last will be closed first, working backward through the list of resources.
 
Winston Gutkowski
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meeta gaur wrote:I couldn't figure out meaning of NOTE: the close() methods of resources are called in the OPPOSITE order of their creation.

Basically, what it means is that close() methods 'drill up' hierarchy-wise, because Closeables (at least the ones in the Java foundation classes) tend to extend or wrap each other.

The example shows a BufferedReader, which requires a ready-made Reader - in your case a FileReader - which in turn extends InputStreamReader which wraps an InputStream.

All the note is telling you is that close() on BufferedReader invokes close() on its FileReader, which in turn calls super.close() (InputStreamReader) which calls the close() method for its InputStream - hence, reverse order of creation.

Winston
 
Campbell Ritchie
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meeta gaur wrote:http://www.javacertifications.net/javacert/Language%20Enhancements.jsp
That appears to be the same as the Java Language Specification (JLS) says. You should always check in the JLS, because it is the definitive guide.
 
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