Developing with Google App Engine (Firstpress)

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Using setter injection eliminates these problems, and so Spring favors that approach. The Guice authors saw difficulties with setter injection. One problem is immutability: it is impossible to make immutable a class that uses setter injection. Constructor injection, on the other hand, makes the creation of immutable classes easy, an important consideration in writing multi-threaded applications. In addition, optional dependencies, while perhaps convenient, can introduce confusion about how a class is configured at runtime.

Configuring a class through setter injection can often lead to missed required dependencies. Though Spring does provide a Required annotation to solve this problem, using constructor injection eliminates it by default. Constructor injection also makes a class's dependencies immediately clear at a glance. If you're writing or modifying a unit test, it's easy to read what the system-under-test needs. Lastly, because Guice uses types to wire classes together, constructor argument order isn't an issue.

You can feel free to reorder them how you want without needing to modify configuration code at all. The potential drawbacks posed by setter injection outweigh the benefits in many common scenarios, and so Guice established a best practice of favoring constructor injection instead.

Its API is well-suited to that approach. But if you should choose to switch from one form of injection to the other, Guice makes that easy too. Changing from setter to constructor injection or vice versa is simply a matter of modifying the class in question. Unlike in Spring, you need never touch a configuration file. Null is easily one of the most non-communicative return values possible from a method call. Its existence means we have to pepper our code--including all our public methods--with is-null checks.

And who can deny the ubiquity of one of the most vexing of Java exceptions: the dreaded NullPointerException? I have spent lots of effort evangelizing colleagues to minimize the use of nulls as much as possible. Guice and Spring both permit nullable and non-nullable injection, and again, each has a preference.

If a bean reference returns a null, Spring has no problem injecting that into other objects. Conversely, Guice hates nulls as much as I do. By default, Guice refuses to inject a null into any object, and if an accidental null shows up during wiring, it fails-fast with a ProvisionException. Guice does allow for the exception case by permitting fields to be annotated with Nullable, but this is discouraged. In fact, the chief complaint I hear about Guice's approach is that it couples application code too tightly to the DI framework. This is a minor concern, in my experience. First, Guice is only "intrusive" as it concerns metadata, and even then, the vast majority of code in a Guice-based application will see only one Guice-specific annotation, Inject.

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If later you decide you want to eliminate Guice from your application, you could do so simply by eliminating the run-time dependency. You would then be free to use Spring, or another DI framework, or even write your own factories. True, it would require leaving a Guice jar in the compile classpath, but you would not need to recurse through the entire code base, ripping out Guice annotations as some have suggested.

If the thought of having Inject sprinkled through your codebase is still unbearable, Guice Provider methods are an alternative very similar to Spring JavaConfig to inject instances without using any Guice-specific annotations. Nonetheless, Spring has also embraced framework-specific annotations, even when using XML config for example, the Required annotation discussed previously.

Moreover, Spring--along with Guice--was an integral member of JSR , which defines a core set of common DI annotations, including javax. Thus, any JSRimplementing framework will be interchangeable, from an application code perspective. Second, switching out DI frameworks is a relatively infrequent activity anyway. Though keeping application code perfectly separate from the framework does make this easier, it may force you to miss out on key features for a questionable tradeoff in benefits.

The Guice authors understood this, and as a result, Guice doesn't advertise itself as a mutually exclusive replacement for Spring. From version 1. Indeed, this complaint seems a little disingenuous coming from the Spring camp in the first place. Yes, a well-configured Spring application has code completely free of references to the framework. But in order to properly maintain a large-scale application in Spring, you really must take advantage of the features of Spring IDE. Does having Inject in your code make you all that more coupled to a framework than dependence on a specific 3rd-party tool for development?

Replacing Spring verbosity with Guicey compactness. The scenario where Guice shines the most and makes its strongest case in comparison to Spring is in configuration complexity when reusing dependencies. Unless you use auto-wiring, which is universally discouraged on any project of scale, Spring requires that all dependencies be explicitly defined.

That means, if you have 50 places where you want to add a dependency, you must modify declarations of that dependency: 50 source files and 50 bean definitions in one or more configuration files. If you later change your mind and want to use a completely different dependency, you must again modify both source and configuration files. This process is error-prone and leads to duplicated effort.

Leveraging Java types, Guice eliminates this extra complexity. If you want to add or delete a dependency from 50 different classes, all you have to do is modify 50 source files, and then modify a single configuration binding. In some cases, such as the injection of concrete classes, you may not need to modify configuration at all.

The use of Java typing again leads to more sensible code, relying on the Java compiler to do the work for you. There are many other benefits Guice offers that give it advantages over Spring. Guice fully supports generics, and has introduced mechanisms such as TypeLiterals and Provider methods to circumvent type erasure. Though Spring offers some basic support for generics by offering type-conversion for collections, its traditional reliance on XML limits its flexibility.

Another excellent Guice advantage is its first-class error messaging. Wiring classes by type and disallowing the injection of nulls gives Guice a distinct advantage at run-time: many errors can be caught at provisioning time, as soon as an injector is created. Guice aggregates these errors and displays them all at once in a clean format, helping you to resolve all configuration problems at once.

Spring, on the other hand, fails after the first wiring error it encounters and provides sometimes cryptic error messages with lengthy stack traces. Worse, in the case of the aforementioned parameterized types, you may not even get an error message until deep into your application. Author and Guice advocate Robbie Vanbrabant demonstrates some of these differences with concrete examples.

Here are some other helpful tips, tricks, tutorials and examples that will show you how to continuously deploy your applications:. In conclusion, although this article may appear to be anti-Spring, I know the framework has value. Spring challenged the J2EE beast, brought dependency injection into the mainstream, and today offers an impressive stack of useful applications built on top of its IoC container.

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It was created in a pre-Java-5 world--a world in which a framework like Guice could not have even existed. The Guice framework takes dependency injection to the next level, leveraging the full power of Java typing, especially annotations and generics, to make powerful DI applications more modular, easier to write, and less error prone to maintain. Ryan Nelson has more than 10 years of software development experience on both large and small projects. New to Git and distributed version control?

Please check the box if you want to proceed. Learn how Oracle was pushing its cloud technologies, but OOW keynote speakers largely talked about cloud strategies. Are you stumped by enterprise cloud applications or related technology? Check out this breakdown of five important terms to help To be a good tester, start at requirements and user expectations, then develop test cases that make certain those needs are met. The new GigaFox platforms enable users to create a pool of shareable mobile devices; remotely add, disable, reboot and unlock Coders can learn a lot from manufacturers, according to Gary Gruver, even though the products are distinctly different.

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