4.7 Working with Dependencies
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## 4.7 Working with Dependencies

Complex enterprise frameworks such a Spring and Hibernate are typically divided into many, many different packages. Traditionally, if an OSGi bundle wished to make extensive use of such a framework its manifest would have to import a huge number of different packages. This can be an error-prone and tedious process. Furthermore, application developers are used to thinking in terms of their application using a framework, such as Spring, as a whole, rather than a long list of all the different packages that comprise the framework.

The following figure provides a simple illustration of the complexity of only using `Import-Package`:

The Virgo Tomcat Server reduces the need for long lists of imported packages by introducing two new manifest headers; `Import-Bundle` and `Import-Library`. The following figure provides an illustration of the simplification that these new headers offer:

As you can see, use of `Import-Bundle` and `Import-Library` can lead to a dramatic reduction in the number of imports that you need to include in an application bundle’s manifest. Furthermore, `Import-Bundle` and `Import-Library` are simply aliases for `Import-Package`; at deployment time `Import-Bundle` and `Import-Library` header entries are automatically expanded into numerous `Import-Package` entries. This means that you retain the exact same semantics of using `Import-Package`, without having to go through the labourious process of doing so.

### Importing Libraries

A bundle in an application can declare a dependency on a library by using the Virgo Server specific `Import-Library` header. This header specifies a comma-separated list of library symbolic names and version ranges that determine which libraries are imported. By default a dependency on a library is mandatory but this can be controlled through use of the resolution directive in exactly the same way as it can with `Import-Package`.

    Import-Library: org.springframework.spring;version="[2.5.4, 3.0)",
     org.aspectj;version="[1.6.0,1.6.0]";resolution:="optional"

This example `Import-Library` header declares a mandatory dependency on the Spring library at a version from 2.5.4 inclusive to 3.0 exclusive. It also declares an optional dependency on the AspectJ library at exactly 1.6.0.

### Importing Bundles

A bundle in an application can declare a dependency on a bundle by using the Virgo Server specific `Import-Bundle` header. The header specifies a comma-separated list of bundle symbolic names, version ranges, and scope declarmations that determine which bundles are imported and the scope of their dependency. By default a dependency on a bundle is mandatory but this can be controlled through use of the resolution directive in exactly the same way as it can with `Import-Package`.

    Import-Bundle: com.springsource.org.apache.commons.dbcp;version="[1.2.2.osgi, 1.2.2.osgi]"

This example `Import-Bundle` header declares a mandatory dependency on the Apache Commons DBCP bundle at exactly 1.2.2.osgi.

#### Disadvantages

A disadvantage of using `Import-Bundle` or especially `Import-Library`, is that the application bundle has a greater apparent fan-out than it strictly needs. An Alternative is to use a tool such as bnd or Bundlor to generate the package imports of the bundle.

### Scoping Bundles in an Application

When working with a scoped application, such as a PAR file or a plan, you might run into a situation where one of the bundles in the application (call it `bundleA`) depends on another bundle (`bundleB`) that performs a runtime task (such as class generation) that a third bundle (`bundleC`) might need to know about, although `bundleC` does not explicitly depend on `bundleB`.

For example, Hibernate uses CGLIB (code generation library) at runtime to generate proxies for persistent classes. Assume that a domain bundle in your application uses Hibernate for its persistent objects, and thus its `Import-Bundle` manifest header includes the Hibernate bundle. Further assume that a separate Web bundle uses reflection in its data-binding code, and thus needs to reflect on the persistent classes generated by Hibernate at runtime. The Web bundle now has an indirect dependency on the Hibernate bundle because of these dynamically generated classes, although the Web bundle does not typically care about the details of how these classes are persisted. One way to solve this dependency problem is to explicitly add the Hibernate bundle to the `Import-Bundle` header of the Web bundle; however, this type of explicit-specified dependency breaks the modularity of the application and is not a programming best practice.

A better way to solve this problem is to specify that Virgo Tomcat Server itself dynamically import the bundle (Hibernate in the example above) to all bundles in the application at runtime. You do this by adding the `import-scope:=application` directive to the `Import-Bundle` header of the bundle that has the direct dependency (the domain bundle in our example). At runtime, although the Web bundle does not explicitly import the Hibernate bundle, Virgo Tomcat Server implicitly imports it and thus its classes are available to the Web bundle. This mechanism allows you to declare the dependencies you need to make your application run, without having to make changes to your application that might limit its flexibility.

The following example shows how to use the `import-scope` directive with the `Import-Bundle` header:

    Import-Bundle: com.springsource.org.hibernate;version="[3.2.6.ga,3.2.6.ga]";import-scope:=application

You can also set the `import-scope` directive to the (default) value `bundle`; in this case, the scope of the bundle is just the bundle itself and thus Virgo Tomcat Server does not perform any implicit importing into other bundles of the application.

Note that use of the `import-scope:=application` directive of the `Import-Bundle` header only makes sense when the bundle is part of a scoped application (PAR or plan); if the bundle is not part of a scoped application, then this directive has no effect.

Finally, because `import-scope:=application` implicitly adds a bundle import to each bundle of the PAR or plan, the impact of subsequently refreshing the imported bundle is, in general, broader than it would have been if you had not used `import-scope:=application`. This may well affect the performance of refresh.

### Defining Libraries

Libraries are defined in a simple text file, typically with a `.libd` suffix. This file identifies the library and lists all of its constituent bundles. For example, the following is the library definition for Spring 2.5.4:

    Library-SymbolicName: org.springframework.spring
    Library-Version: 2.5.4
    Library-Name: Spring Framework
    Import-Bundle: org.springframework.core;version="[2.5.4,2.5.5)",
     org.springframework.beans;version="[2.5.4,2.5.5)",
     org.springframework.context;version="[2.5.4,2.5.5)",
     org.springframework.aop;version="[2.5.4,2.5.5)",
     org.springframework.web;version="[2.5.4,2.5.5)",
     org.springframework.web.servlet;version="[2.5.4,2.5.5)",
     org.springframework.jdbc;version="[2.5.4,2.5.5)",
     org.springframework.orm;version="[2.5.4,2.5.5)",
     org.springframework.transaction;version="[2.5.4,2.5.5)",
     org.springframework.context.support;version="[2.5.4,2.5.5)",
     org.springframework.aspects;version="[2.5.4,2.5.5)",
     com.springsource.org.aopalliance;version="1.0"

The following table lists all of the headers that may be used in a library definition:

Table 4.4. Library definition headers

Header | Description
--- | ---
Library-SymbolicName | Identifier for the library
Library-Version | Version number for the library
Import-Bundle | A comma separated list of bundle symbolic names. Each entry may optionally specify a version (using the `version=` directive) and the scope of the import (using the `import-scope` directive).
Library-Name | Optional. The human-readable name of the library
Library-Description | Optional. A human-readable description of the library

### Installing Dependencies

Rather than encouraging the packaging of all an application’s dependencies within the application itself, Virgo Tomcat Server uses a local provisioning repository of bundles and libraries upon which an application can depend. When the Virgo Tomcat Server encounters an application with a particular dependency, it will automatically provide, from its provisioning repository, the appropriate bundle or library.

Making a dependency available for provisioning is simply a matter of copying it to the appropriate location in the VTS’s local provisioning repository. By default this is `SERVER_HOME/repository/usr`. A more detailed discussion of the provisioning repository can be found in the User Guide.

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