OSGi doesn’t allow you to specify class visibility at the class level, but instead at the level of the containing package.
1. The header：Export-Package and The directives for the Export-Package header
当指定一个package export，默认的它的subpackage是没有exportde,需特别指定。
Directives for the Export-Package header：
(1)exclude : Specifies a list of packages that must be invisible to the component importing the
package.
(2)include : Specifies a list of packages that must be visible to the component importing the
package.
(3)mandatory： Specifies a list of mandatory properties that must be specified when using the
exported package in other components.
(4)uses : Specifies a list of packages that an exported package uses. This directive is used by
the dependency resolver to ensure that imported package versions correspond to those
specified here.
默认情况下，在Export Package是，OSGi框架会自动将ExportBundles的bundle-symbolic-name和Bundle
-Version属性添加上。
例如：Export-Package:
org.springframework.beans.factory.serviceloader;
uses:="org.springframework.util,
org.springframework.beans. factory.config,
org.springframework.beans.factory";
version=2.5.5,
org.springframework.beans.annotation;
uses:="org.springframework.util,
org.springframework.beans";
version=2.5.5,
...
2. The header:Import-Package and It's stand attributes
Standard attributes usable with the Import-Package header:
(1)version : Specifies the version range for matching exported packages. If this attribute
isn’t specified, the default value is [0.0.0, ∞).
(2)bundle-symbolic-name : Specifies the symbolic name of the component that exports the
packages. In the case of a fragment bundle, the name will correspond to the host bundle’s
symbolic name instead. A fragment is a special bundle that allows extending an existing
bundle.
(3)bundle-version : Specifies the version of the component that exports the packages. As
with version, the default value is [0.0.0, ∞) and the fragment behavior is as for bundle-
symbolic-name.
例如：
Import-Package: javax.el;version="[2.1.0, 3.0.0)";resolution:=optional
net.sf.cglib.proxy;version="[2.1.3, 2.2.0)";resolution:=optional,
org.apache.commons.logging;version="[1.0.4, 2.0.0)",
org.springframework.core;version="[2.5.6, 2.5.6]";resolution:=optional,
org.springframework.core.annotation;version="[2.5.6, 2.5.6]";
resolution:=optional
In addition to the previous attribute, the resolution directive of the Import-Package header
makes it possible to change the behavior of this resolution mechanism.The first possible value
of the directive is mandatory, which is also the default.When a package is marked as
mandatory, it means that if the package can’t be resolved, the component itself can’t be
resolved. The other value is optional. In this case, if the package isn’t present during the
resolution phase, the package isn’t considered imported by the component.
3. The header: DynamicImport-Package
It bypass the regular class-resolution mechanism we have described and instead allows
classes to be loaded on demand at runtime. Dependencies specified in this way are ignored
during the transition between the installed and resolved states.
The DynamicImport-Package header can contain a list of packages, but it also supports two
wildcards for specifying package names. Moreover, these imports are searched for in the order
in which they are specified. This ordering is important, especially when using wildcards,
because it determines the order in which matching occurs. In the following snippet, packages
with the value test for the attribute myattribute will be preferred over other packages.
DynamicImport-Package: *;myattribute=test, *
No directives are defined for this header, but the standard version, bundlesymbolic-name,
and bundle-version attributes can all be used for package matching with the same semantics
as Import-Package.
4. The header: Require-Bundle
The Require-Bundle header is provided by the OSGi specification so you can consume all
exported packages from other components based on component symbolic names.
The Require-Bundle only has one attribute:bundle-version,the bundle-version attribute uses
the same semantics as the version attribute of the Import-Package header and specifies
the component version of the dependency.
例如：
Manifest-Version: 1.0
(...)
Import-Package: javax.servlet;version="2.5.0",
javax.servlet.http;version="2.5.0",
javax.servlet.jsp;version="2.0.0",
(...)
Require-Bundle: com.springsource.javax.servlet.jsp.jstl;
bundle-version=1.1.2,com.springsource.org.apache.taglibs.standard;
bundle-version=1.1.2
注意：同一份MANIFEST.MF配置文件中，Import-Package header takes priority over the Require
-Bundle header.
Directives usable with the Require-Bundle header：
(1)visibility : When set to private (the default value), the exported packages of the
corresponding bundles aren’t re-exported. Setting this to reexport will achieve the opposite.
(2)resolution : Controls dependency resolution. If set to mandatory (the default value), the
dependency must be successfully resolved for the bundle to resolve. With the optional value,
dependencies are only resolved if they’re present, and the bundle will resolve regardless.
5. Split packages
You saw that referencing bundles with the Require-Bundle header is equivalent to importing
all of their exported packages. A problematic case occurs when two different required bundles
export the same package. Such packages are named split packages because their content
comes from several bundles.
Although OSGi allows split packages, you’ll notice that they have several obvious drawbacks.
When different bundles provide the same classes, unpredictable shadowing of classes can arise.
6. Matching and Versioning
6.1 Version Matching
The different parts of a version value:
(1)Major number : For major updates with no provision for compatibility with previous major
versions.
(2)Minor number : For functional updates compatible with the current major version (compatible
for clients using an interface, but not for implementers).
(3)Micro number : For bug fixes.
(4)Qualifier : Values like “–SNAPSHOT” for the current development version can also be added
at the end of the value. The version number with a qualifier is lower than the number without.
6.2 Optional Depedencies
OSGi supports optional dependencies on both packages and bundles with the Import-Package
and Require-Bundle headers.
6.3 Attribute Matching
模式：
Header-Name: header-value1;directive-name1:=directive-value1;attribute-name1= attribute
-value1;attribute-name2=attribute-value2,
(...)
例如：
Export-Package: com.manning.osgi.simple;criteria="Manning";mandatory:="criteria"
Import-Package: com.manning.osgi.simple;criteria="Manning"