Java Context Introduction and Example
This interface represents a naming context, which consists of a set of name-to-object bindings.
It contains methods for examining and updating these bindings.
Names
Each name passed as an argument to a Context method is relative to that context.
The empty name is used to name the context itself.
A name parameter may never be null.
Most of the methods have overloaded versions with one taking a Name parameter and one taking a String.
These overloaded versions are equivalent in that if the Name and String parameters are just different representations of the same name, then the overloaded versions of the same methods behave the same.
In the method descriptions below, only one version is fully documented.
The second version instead has a link to the first: the same documentation applies to both.
For systems that support federation, String name arguments to Context methods are composite names.
Name arguments that are instances of CompositeName are treated as composite names, while Name arguments that are not instances of CompositeName are treated as compound names (which might be instances of CompoundName or other implementations of compound names).
This allows the results of NameParser.parse() to be used as arguments to the Context methods.
Prior to JNDI 1.2, all name arguments were treated as composite names.
Furthermore, for systems that support federation, all names returned in a NamingEnumeration from list() and listBindings() are composite names represented as strings.
See CompositeName for the string syntax of names.
For systems that do not support federation, the name arguments (in either Name or String forms) and the names returned in NamingEnumeration may be names in their own namespace rather than names in a composite namespace, at the discretion of the service provider.
Exceptions
All the methods in this interface can throw a NamingException or any of its subclasses.
See NamingException and their subclasses for details on each exception.
Concurrent Access
A Context instance is not guaranteed to be synchronized against concurrent access by multiple threads.
Threads that need to access a single Context instance concurrently should synchronize amongst themselves and provide the necessary locking.
Multiple threads each manipulating a different Context instance need not synchronize.
Note that the #lookup(Name) lookup method, when passed an empty name, will return a new Context instance representing the same naming context.
For purposes of concurrency control, a Context operation that returns a NamingEnumeration is not considered to have completed while the enumeration is still in use, or while any referrals generated by that operation are still being followed.
Parameters
A Name parameter passed to any method of the Context interface or one of its subinterfaces will not be modified by the service provider.
The service provider may keep a reference to it for the duration of the operation, including any enumeration of the method’s results and the processing of any referrals generated.
The caller should not modify the object during this time.
A Name returned by any such method is owned by the caller.
The caller may subsequently modify it; the service provider may not.
Environment Properties
JNDI applications need a way to communicate various preferences and properties that define the environment in which naming and directory services are accessed.
For example, a context might require specification of security credentials in order to access the service.
Another context might require that server configuration information be supplied.
These are referred to as the environment of a context.
The Context interface provides methods for retrieving and updating this environment.
The environment is inherited from the parent context as context methods proceed from one context to the next.
Changes to the environment of one context do not directly affect those of other contexts.
It is implementation-dependent when environment properties are used and/or verified for validity.
For example, some of the security-related properties are used by service providers to «log in» to the directory.
This login process might occur at the time the context is created, or the first time a method is invoked on the context.
When, and whether this occurs at all, is implementation-dependent.
When environment properties are added or removed from the context, verifying the validity of the changes is again implementation-dependent.
For example, verification of some properties might occur at the time the change is made, or at the time the next operation is performed on the context, or not at all.
Any object with a reference to a context may examine that context’s environment.
Sensitive information such as clear-text passwords should not be stored there unless the implementation is known to protect it.
Resource Files
To simplify the task of setting up the environment required by a JNDI application, application components and service providers may be distributed along with resource files.
A JNDI resource file is a file in the properties file format (see java.util.Properties#load java.util.Properties), containing a list of key/value pairs.
The key is the name of the property (e.g. «java.naming.factory.object») and the value is a string in the format defined for that property.
Here is an example of a JNDI resource file: java.naming.factory.object=com.sun.jndi.ldap.AttrsToCorba:com.wiz.from.Person java.naming.factory.state=com.sun.jndi.ldap.CorbaToAttrs:com.wiz.from.Person java.naming.factory.control=com.sun.jndi.ldap.ResponseControlFactory The JNDI class library reads the resource files and makes the property values freely available.
Thus JNDI resource files should be considered to be «world readable», and sensitive information such as clear-text passwords should not be stored there.
There are two kinds of JNDI resource files: provider and application.
Provider Resource Files
Each service provider has an optional resource that lists properties specific to that provider.
The name of this resource is: [prefix/]jndiprovider.properties where prefix is the package name of the provider’s context implementation(s), with each period («.») converted to a slash («/»).
For example, suppose a service provider defines a context implementation with class name com.sun.jndi.ldap.LdapCtx.
The provider resource for this provider is named com/sun/jndi/ldap/jndiprovider.properties.
If the class is not in a package, the resource’s name is simply jndiprovider.properties.
Certain methods in the JNDI class library make use of the standard JNDI properties that specify lists of JNDI factories:
- java.naming.factory.object
- java.naming.factory.state
- java.naming.factory.control
- java.naming.factory.url.pkgs
The JNDI library will consult the provider resource file when determining the values of these properties.
Properties other than these may be set in the provider resource file at the discretion of the service provider.
The service provider’s documentation should clearly state which properties are allowed; other properties in the file will be ignored.
Application Resource Files
When an application is deployed, it will generally have several codebase directories and JARs in its classpath.
JNDI locates (using ClassLoader#getResources ClassLoader.getResources()) all application resource files named jndi.properties in the classpath.
In addition, if the Java installation directory contains a built-in properties file, typically conf/jndi.properties, JNDI treats it as an additional application resource file.
All of the properties contained in these files are placed into the environment of the initial context.
This environment is then inherited by other contexts.
For each property found in more than one application resource file, JNDI uses the first value found or, in a few cases where it makes sense to do so, it concatenates all of the values (details are given below).
For example, if the «java.naming.factory.object» property is found in three jndi.properties resource files, the list of object factories is a concatenation of the property values from all three files.
Using this scheme, each deployable component is responsible for listing the factories that it exports.
JNDI automatically collects and uses all of these export lists when searching for factory classes.
Search Algorithm for Properties
When JNDI constructs an initial context, the context’s environment is initialized with properties defined in the environment parameter passed to the constructor, the system properties, and the application resource files.
See InitialContext for details.
This initial environment is then inherited by other context instances.
When the JNDI class library needs to determine the value of a property, it does so by merging the values from the following two sources, in order:
- The environment of the context being operated on.
- The provider resource file (jndiprovider.properties) for the context being operated on.
For each property found in both of these two sources, JNDI determines the property’s value as follows.
If the property is one of the standard JNDI properties that specify a list of JNDI factories (listed above), the values are concatenated into a single colon-separated list.
For other properties, only the first value found is used.
When a service provider needs to determine the value of a property, it will generally take that value directly from the environment.
A service provider may define provider-specific properties to be placed in its own provider resource file.
In that case it should merge values as described in the previous paragraph.
In this way, each service provider developer can specify a list of factories to use with that service provider.
These can be modified by the application resources specified by the deployer of the application, which in turn can be modified by the user.
Example
The following code shows how to use Context from javax.naming.
import java.util.Hashtable; import javax.naming.Context; import javax.naming.InitialContext; public class Main < public static void main(String[] argv) throws Exception < String url = "iiop://localhost/"; Hashtable env = new Hashtable(); env.put(Context.INITIAL_CONTEXT_FACTORY, "com.sun.jndi.cosnaming.CNCtxFactory"); env.put(Context.PROVIDER_URL, url); Context ctx = new InitialContext(env); > >
import java.util.Hashtable; import javax.naming.Context; import javax.naming.directory.DirContext; import javax.naming.directory.InitialDirContext; public class Main < public static void main(String[] argv) throws Exception < String url = "ldap://localhost/o=JNDITutorial"; Hashtable env = new Hashtable(); env.put(Context.INITIAL_CONTEXT_FACTORY, "com.sun.jndi.ldap.LdapCtxFactory"); env.put(Context.PROVIDER_URL, url); DirContext ctx = new InitialDirContext(env); > >
import javax.naming.Context; import javax.naming.NamingException; import javax.naming.directory.DirContext; import javax.naming.directory.InitialDirContext; import java.util.Hashtable; public class Main < public static void main(String[] args) throws NamingException < HashtableString, Object> env = new HashtableString, Object>(); env.put(Context.INITIAL_CONTEXT_FACTORY, "com.sun.jndi.ldap.LdapCtxFactory"); env.put(Context.PROVIDER_URL, "ldap://localhost:389/dc=fraglab,dc=net"); env.put(Context.SECURITY_AUTHENTICATION, "none"); DirContext ctx = new InitialDirContext(env); ctx.close(); > >
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Context object
Decouple data from protocol-specific classes and store the scoped data in an object independent of the underlying protocol technology.
# Explanation
This application has different layers labelled A, B and C with each extracting specific information from a similar context for further use in the software. Passing down each pieces of information individually would be inefficient, a method to efficiently store and pass information is needed.
Create an object and store the data there and pass this object to where it is needed.
Use a Context Object to encapsulate state in a protocol-independent way to be shared throughout your application.
Programmatic Example
We define what data a service context object contains.
public class ServiceContext < String ACCOUNT_SERVICE, SESSION_SERVICE, SEARCH_SERVICE; public void setACCOUNT_SERVICE(String ACCOUNT_SERVICE) < this.ACCOUNT_SERVICE = ACCOUNT_SERVICE; >public void setSESSION_SERVICE(String SESSION_SERVICE) < this.SESSION_SERVICE = SESSION_SERVICE; >public void setSEARCH_SERVICE(String SEARCH_SERVICE) < this.SEARCH_SERVICE = SEARCH_SERVICE; >public String getACCOUNT_SERVICE() < return ACCOUNT_SERVICE; >public String getSESSION_SERVICE() < return SESSION_SERVICE; >public String getSEARCH_SERVICE() < return SEARCH_SERVICE; >public String toString() < return ACCOUNT_SERVICE + " " + SESSION_SERVICE + " " + SEARCH_SERVICE;>>