Release resources in java

Releasing Resources In Java

Many objects hold references to system resources (files, memory, database locks, etc.) which need to be released in a timely fashion when they are no longer needed.

In C++, the ResourceAcquisitionIsInitialization idiom is widely used for this, which works because of explicit object lifetimes (objects have destructors which are called when they go out of scope or are deleted; those can release resources at that time).

Java automates the reclamation of memory, but not other resources. A finalize() method can help ensure that resources are released eventually but not in a timely fashion. It appears that this has been handled ad-hoc in various Java classes — some have close(), some dispose(), some destroy(), and probably others.

I haven’t come up with an elegant general solution to this, and wonder if anyone else has.

— JimPerry I think this is a concern for all objects with semantically determined lifetimes. I suggest, therefore, that we consider this as a specialization of ReleasingResourcesOfObjects.

TomStambaugh A first step is to use the finalize() method detect errors, for example by asserting(false) (if you have an assert mechanism).

If an attempt to allocate a resource fails, it may be worth requesting a GC and calling finalizers, and trying again.

Slightly more interesting is to protect your resource with inner classes. Like:

interface Closure < void exec(); >

class Resource < private abstract void acquire(); private abstract void release();

public void withDo( Closure c ) < acquire(); try < c.exec(); >finally < release(); >> > void test( Resource r ) < r.withDo ( new Closure() < void exec() < System.out.println( r ); // or whatever >> ); >

The example by DaveHarris above is similar to one I came up with a couple of days ago in a discussion in the Comp.object news group. The difference is that ResourceReleasesResource attempts to be abstract and reusable by multiple resource providers. Any object can be a resource client if it implements the ResourceClient interface. (This seems more general than an inner class.)

-PatrickLogan For what it’s worth, standard practice in Java is to give the resource a close() method and call it within a finally clause. (See: java.io, java.net, java.sql.) The use of a Closure inner class seems like a nice way to do too, but you’d need to write your own adapter classes.

-BrianSlesinsky Finalizers don’t actually ensure that your resources will be eventually freed. Some VM’s don’t call finalizers on exit by default, and some refuse to at all.. plus there’s always the chance you’ll crash the VM (I average about a crash a week.. 98% of them are my fault). Furthermore, be careful what you expect out of gc() and runFinalization().. Different VM’s act very differently, and some seem to act unpredictably with large numbers (ie >10,000) of free objects. For reference, HotSpot seems to be quite predictable, and gc()’s free objects very quickly.. however, that isn’t the behavior I want for my current project, so I’m using Symantec’s JVM.

To me, close(), destroy(), etc have different meanings. In most cases, I think close() is appropriate (open files, sockets, db connections, etc). I do have cases where I use something else instead. For instance, destroy() to me means that some object (ie an object that has a temporary file) actually has to go out and free resources from somewhere else.

CategoryJava EditText of this page (last edited January 29, 2003)
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Is this a safe way to release resources in Java?

For more information, check https://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html Solution 2: As far as I know, it’s compulsory to declare the resource in the try-with-resources block as it’s done in the second example. Question: The block is mainly used to prevent resource leaks which can be achieved in method of resource class.

Is this a safe way to release resources in Java?

Usually when code needs some resource that needs to be released I see it done like this:

InputStream in = null; try< in = new FileInputStream("myfile.txt"); doSomethingWithStream(in); >finally < if(in != null)< in.close(); >> 

What I don’t like is that you have to initialize the variable to null and after that set it to another value and in the finally block check if the resource was initialized by checking if it is null. If it is not null, it needs to be released. I know I’m nitpicking, but I feel like this could be done cleaner.

What I would like to do is this:

InputStream in = new FileInputStream("myfile.txt"); try< doSomethingWithStream(in); >finally

To my eyes this looks almost as safe as the previous one. If resource initialization fails and it throws an exception, there’s nothing to be done(since I didn’t get the resource) so it doesn’t have to be inside the try block. The only thing I’m worried is if there is some way(I’m not Java certified) that an exception or error can be thrown between operations?

Inputstream in = new FileInputStream("myfile.txt"); in.close(); 

Is there any way the stream would be left open that a try-finally block would prevent?

Maybe I should have left out the last example because it confuses everyone. This is not supposed to be a beginner level question. I know what try-finally does and I know that it is not safe if there was doSomethingWithStream in the middle in the last example. That is why it is not there. I accepted Eyals answer because it was precisely what I was looking for. There is a way to cause an exception between two operations, which would make the middle example unsafe(using Thread.stop), but since it is made using deprecated call and can mess you up no matter what you do so I feel safe using the middle example.

The middle sample is safe — the last one isn’t.

A try-finally block means that the stream is closed even if doSomethingWithStream throws an exception. Yes, you could catch all exceptions and then close the stream that way — but it’s much simpler to let the exception bubble up to the caller, but close the stream on the way via the finally block.

Actually, an exception can occur between the 2 calls in the last code sample,and leave the resource open. If the thread is stopped violently by another thread using the Thread.stop() or Thread.stop(Throwable) immediately after the stream construction, the thread will throw an exception (ThreadDeath in the first case), and the resources will not be disposed of.

But this is exactly why these methods are deprecated.

Look into Project Lombok. It has a @Cleanup annotation which is settable on local variables which will auto-generate code at compilation time to clean up the resource.

 import lombok.Cleanup; import java.io.*; public class CleanupExample < public static void main(String[] args) throws IOException < @Cleanup InputStream in = new FileInputStream(args[0]); @Cleanup OutputStream out = new FileOutputStream(args[1]); byte[] b = new byte[10000]; while (true) < int r = in.read(b); if (r == -1) break; out.write(b, 0, r); >> > 

Using return in try block (Java), here the values are returned from try block and finally block. First it will execute the try block but before return it should execute the finally block. So this method will return 101, from the finally block instead of 100. So returning from the finally block might produce unexpected results for the casual reader. In my view

What’s the use of finally block in try with resources(Java 7)?

The finally block is mainly used to prevent resource leaks which can be achieved in close() method of resource class. What’s the use of finally block with try-with-resources statement, e.g:

class MultipleResources < class Lamb implements AutoCloseable < public void close() throws Exception < System.out.print("l"); >> class Goat implements AutoCloseable < public void close() throws Exception < System.out.print("g"); >> public static void main(String[] args) throws Exception < new MultipleResources().run(); >public void run() throws Exception < try (Lamb l = new Lamb(); Goat g = new Goat();) < System.out.print("2"); >finally < System.out.print("f"); >> > 

Ref: K.Seirra, B. Bates OCPJP Book

Just like in regular try-catch-finally block — finally block is used when you want something to always happen, no matter if operation in try block succeeds or not.

I think your question is about providing some use-case when it is really useful. Try to imagine a situation when you have to tell one collabolator (or publish an event) that your processing is finished — regardless of its result. You can then put in the finally block the code which is resposible for announcing the finishing of processing.

Plase note that when some operation in try-with-resources block without catch throws an exception, the code following that block will not be executed.

The code in finally block with always get executed unless the thread that executing the code or the JVM is terminated.

This ensures that any resources you allocated will get cleaned up in the finally regardless.

The Java doc has a detail explanation of finally.

Java try-catch, Java try block Java try block is used to enclose the code that might throw an exception. It must be used within the method. If an exception occurs at the particular statement in the try block, the rest of the block code will not execute. So, it is recommended not to keep the code in try block that will not throw an …

Why is Java complaining about my try-with-resources block?

I’ve got an IncomingTrackHandlerFactory ( ith ) which hands out instances of IncomingTrackHandler . These instances implements AutoCloseable . The IncomingTrackHandler deals with the database, and is short lived. Every instance is just used for a couple of queries and then discarded.

I don’t get why the first block of code does not work. Why does Java tell me that it «cannot find symbol» for ith ? I’m simply just declaring ith before the try block to be able to also have the ith variable at hand if an exception is caugth and the database transaction must be rolled back.

Your ith object is only visible inside your try block and is an instance of AutoCloseable . The instance is not visible outside, neither in catch, finally or catch block. The ith resource is closed automatically when an exception is thrown or the try block is completed. Although you can catch the exception itself, but can not operate on the ith object itself, only inside the try block.

To use the rollback function, you have to declare another try-catch block inside it. (Original code example omitted due source code is in image format)

try(Object smth = source.get()) < try < // operate on smth >catch (Exception e) < smth.rollback(); >> 

For more information, check https://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html

As far as I know, it’s compulsory to declare the resource in the try-with-resources block as it’s done in the second example.

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).

More info here: https://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html

Java 9 Try-With Resources, Java introduced try-with-resource feature in Java 7 that helps to close resource automatically after being used. In other words, we can say that we don’t need to close resources (file, connection, network etc) explicitly, try-with-resource close that automatically by using AutoClosable interface.

Does try with resources execute finally block before catch? [duplicate]

I learned java 7 feature try with resources .

In catch block I should handle exceptions from try (as usually) and from finally

Does it mean that finally was invoked before catch? What if I want to treat to Closeable resource in catch?

There is one observation, which confirms my assumption.

If try block throw exception1 and close method of resource throws exception2 then exception1 will catched and exception2 will supressed.

What if I want to treat to Closeable resource in catch?

Resources declared in the try part have their scope restricted to the try body. If you need access to them in the catch , use a regular try-catch statement.

The finally block will always execute last.

Try-with-resources Feature in Java, In Java, the Try-with-resources statement is a try statement that declares one or more resources in it. A resource is an object that must be closed once your program is done using it. For example, a File resource or a Socket connection resource.

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