c# create barcode image B.2.4 Reflection in Java

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B.2.4 Reflection
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Runtime reflection has an image problem in the Java community: Reflective systems are perceived to be slow. This is partly untrue reflection is much faster in modern JVMs than in JDK 1.2 and partly irrelevant to our class of problems. Even if reflection was really as slow as some people assume, the overhead is insignificant compared to the cost of disk access and interprocess (even network) communication that dominates data access. There really is no good reason to avoid the use of reflection in a persistence layer. On the other hand, there is one excellent reason to choose reflection over other techniques such as code generation: Reflection doesn t intrude on the build cycle
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APPENDIX B
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or on the performance of system initialization. This has a definite positive effect on developer performance. Early versions of Hibernate used reflection exclusively for interaction with the domain objects. This solution was popular with users and was shown to cause only a slight performance overhead. However, JDK reflection was found insufficient when it came to the problem of lazy fetching. To implement lazy fetching of associations, Hibernate uses proxies. A proxy is an object that implements the public interface of a business object and intercepts all messages sent to that object by its clients. In the case of Hibernate, interception is used to load the proxied object s state from the database the first time it s used. For proxying many-valued associations, Hibernate uses implementations of the collection interfaces defined in java.util. For single-point associations (an object reference to a user-defined class), more sophisticated machinery is required. Java provides java.lang.reflect.Proxy for JDK 1.3 and above. An instance of Proxy may be instantiated at runtime, implementing a given list of interfaces. This solution is great if your persistent objects are accessed by clients only via an interface, but what if the persistent class doesn t implement an interface at all You don t want to force the unwieldy EJB-style local interfaces on your POJOs. They re supposed to be plain, remember! So, if Java reflection can t solve all your problems, what option remains
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B.2.5 Runtime bytecode generation
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Fortunately, exactly when the Hibernate team needed it, another open source project came along and neatly solved this problem. The CGLIB project describes itself as a code generation library, but we prefer to think of it as an alternative reflection package for Java a replacement for java.lang.reflect. CGLIB uses runtime bytecode generation to implement some of the same features provided by the Java reflection API, only more efficiently. Most important, CGLIB can create proxies that inherit a class, as well as implement interfaces! This allows Hibernate to implement lazy association fetching almost completely transparently. At runtime, clients may hold a reference to a proxy that is a generated subclass of the associated persistent class. When the client invokes a method of the proxy, Hibernate intercepts that method call and loads the state of the proxied object from the database. For classes with all nonprivate property accessor methods, CGLIB also can be used to bypass reflection and get/set property values with normal Java method invocation (in generated bytecode). Hibernate uses this feature whenever possible.
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APPENDIX B
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Some Hibernate users found that the bytecode generation step, which occurs at system-initialization time, was slow in earlier versions of Hibernate. The current release of Hibernate integrates CGLIB 2.0, which vastly improves the performance of bytecode generation and helps reduce the startup time of the Hibernate application. CGLIB is an amazingly useful library. If you have generic programming problems, we encourage you to see if CGLIB can help.
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B.2.6 Generic objects
Some persistence layers, which we hesitate to consider ORM implementations, persist objects consisting of a dynamic set of properties collections of name/value pairs. (Some even encourage you to extend the dynamic class with application classes that add typesafe property access.) We won t pay much attention to this kind of approach since we re most interested in persistence for POJOs. However, this approach fills its own important niche: metadata-driven applications. Typesafe domain models aren t appropriate for applications where the business model is defined in metadata. A great example of this approach is the OFBiz entity engine. But Hibernate 3 (which is in an early stage of development at the time of this writing) supports this kind of application by allowing domain models to be represented as a graph of Maps containing property name/value pairs.
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