# Technical Overview

## Runtime Environment

Spring Security is written to execute within a standard Java 1.4 Runtime Environment. It also supports Java 5.0, although the Java types which are specific to this release are packaged in a separate package with the suffix "tiger" in their JAR filename. As Spring Security aims to operate in a self-contained manner, there is no need to place any special configuration files into your Java Runtime Environment. In particular, there is no need to configure a special Java Authentication and Authorization Service (JAAS) policy file or place Spring Security into common classpath locations.

Similarly, if you are using an EJB Container or Servlet Container there is no need to put any special configuration files anywhere, nor include Spring Security in a server classloader.

This design offers maximum deployment time flexibility, as you can simply copy your target artifact (be it a JAR, WAR or EAR) from one system to another and it will immediately work.

## Shared Components

Let's explore some of the most important shared components in Spring Security. Components are considered "shared" if they are central to the framework and the framework cannot operate without them. These Java types represent the building blocks of the remaining system, so it's important to understand that they're there, even if you don't need to directly interact with them.

### SecurityContextHolder, SecurityContext and Authentication Objects

The most fundamental object is SecurityContextHolder. This is where we store details of the present security context of the application, which includes details of the principal currently using the application. By default the SecurityContextHolder uses a `ThreadLocal` to store these details, which means that the security context is always available to methods in the same thread of execution, even if the security context is not explicitly passed around as an argument to those methods. Using a `ThreadLocal` in this way is quite safe if care is taken to clear the thread after the present principal's request is processed. Of course, Spring Security takes care of this for you automatically so there is no need to worry about it.

Some applications aren't entirely suitable for using a `ThreadLocal`, because of the specific way they work with threads. For example, a Swing client might want all threads in a Java Virtual Machine to use the same security context. For this situation you would use the `SecurityContextHolder.MODE_GLOBAL`. Other applications might want to have threads spawned by the secure thread also assume the same security identity. This is achieved by using `SecurityContextHolder.MODE_INHERITABLETHREADLOCAL`. You can change the mode from the default `SecurityContextHolder.MODE_THREADLOCAL` in two ways. The first is to set a system property. Alternatively, call a static method on SecurityContextHolder. Most applications won't need to change from the default, but if you do, take a look at the JavaDocs for SecurityContextHolder to learn more.

Inside the SecurityContextHolder we store details of the principal currently interacting with the application. Spring Security uses an Authentication object to represent this information. Whilst you won't normally need to create an Authentication object yourself, it is fairly common for users to query the Authentication object. You can use the following code block - from anywhere in your application - to obtain the name of the authenticated user, for example:

    Object obj = SecurityContextHolder.getContext().getAuthentication().getPrincipal();

    if (obj instanceof UserDetails) {
      String username = ((UserDetails)obj).getUsername();
    } else {
      String username = obj.toString();
    }

The above code introduces a number of interesting relationships and key objects. First, you will notice that there is an intermediate object between SecurityContextHolder and Authentication. The `SecurityContextHolder.getContext()` method is actually returning a SecurityContext.

### The UserDetailsService

Another item to note from the above code fragment is that you can obtain a principal from the Authentication object. The principal is just an `Object`. Most of the time this can be cast into a UserDetails object. UserDetails is a central interface in Spring Security. It represents a principal, but in an extensible and application-specific way. Think of UserDetails as the adapter between your own user database and what Spring Security needs inside the SecurityContextHolder. Being a representation of something from your own user database, quite often you will cast the UserDetails to the original object that your application provided, so you can call business-specific methods (like `getEmail()`, `getEmployeeNumber()` and so on).

By now you're probably wondering, so when do I provide a UserDetails object? How do I do that? I thought you said this thing was declarative and I didn't need to write any Java code - what gives? The short answer is that there is a special interface called UserDetailsService. The only method on this interface accepts a `String`-based username argument and returns a UserDetails. Most authentication providers that ship with Spring Security delegate to a UserDetailsService as part of the authentication process. The UserDetailsService is used to build the Authentication object that is stored in the SecurityContextHolder. The good news is that we provide a number of UserDetailsService implementations, including one that uses an in-memory map and another that uses JDBC. Most users tend to write their own, though, with such implementations often simply sitting on top of an existing Data Access Object (DAO) that represents their employees, customers, or other users of the enterprise application. Remember the advantage that whatever your UserDetailsService returns can always be obtained from the SecurityContextHolder, as per the above code fragment.

### GrantedAuthority

Besides the principal, another important method provided by Authentication is `getAuthorities(`). This method provides an array of GrantedAuthority objects. A GrantedAuthority is, not surprisingly, an authority that is granted to the principal. Such authorities are usually "roles", such as `ROLE_ADMINISTRATOR` or `ROLE_HR_SUPERVISOR`. These roles are later on configured for web authorization, method authorization and domain object authorization. Other parts of Spring Security are capable of interpreting these authorities, and expect them to be present. GrantedAuthority objects are usually loaded by the UserDetailsService.

Usually the GrantedAuthority objects are application-wide permissions. They are not specific to a given domain object. Thus, you wouldn't likely have a GrantedAuthority to represent a permission to `Employee` object number 54, because if there are thousands of such authorities you would quickly run out of memory (or, at the very least, cause the application to take a long time to authenticate a user). Of course, Spring Security is expressly designed to handle this common requirement, but you'd instead use the project's domain object security capabilities for this purpose.

Last but not least, sometimes you will need to store the SecurityContext between HTTP requests. Other times the principal will re-authenticate on every request, although most of the time it will be stored. The HttpSessionContextIntegrationFilter is responsible for storing a SecurityContext between HTTP requests. As suggested by the name of the class, the `HttpSession` is used to store this information. You should never interact directly with the `HttpSession` for security purposes. There is simply no justification for doing so - always use the SecurityContextHolder instead.

### Summary

Just to recap, the major building blocks of Spring Security are:

- SecurityContextHolder, to provide any type access to the SecurityContext.

- SecurityContext, to hold the Authentication and possibly request-specific security information.

- HttpSessionContextIntegrationFilter, to store the SecurityContext in the `HttpSession` between web requests.

- Authentication, to represent the principal in a Spring Security-specific manner.

- GrantedAuthority, to reflect the application-wide permissions granted to a principal.

- UserDetails, to provide the necessary information to build an Authentication object from your application's DAOs.

- UserDetailsService, to create a UserDetails when passed in a `String`-based username (or certificate ID or alike).

Now that you've gained an understanding of these repeatedly-used components, let's take a closer look at the process of authentication.

## Authentication

As mentioned in the beginning of this reference guide, Spring Security can participate in many different authentication environments. Whilst we recommend people use Spring Security for authentication and not integrate with existing Container Managed Authentication, it is nevertheless supported - as is integrating with your own proprietary authentication system. Let's first explore authentication from the perspective of Spring Security managing web security entirely on its own, which is illustrative of the most complex and most common situation.

Consider a typical web application's authentication process:

1. You visit the home page, and click on a link.

2. A request goes to the server, and the server decides that you've asked for a protected resource.

3. As you're not presently authenticated, the server sends back a response indicating that you must authenticate. The response will either be an HTTP response code, or a redirect to a particular web page.

4. Depending on the authentication mechanism, your browser will either redirect to the specific web page so that you can fill out the form, or the browser will somehow retrieve your identity (eg a BASIC authentication dialogue box, a cookie, a X509 certificate etc).

5. The browser will send back a response to the server. This will either be an HTTP POST containing the contents of the form that you filled out, or an HTTP header containing your authentication details.

6. Next the server will decide whether or not the presented credentials are valid. If they're valid, the next step will happen. If they're invalid, usually your browser will be asked to try again (so you return to step two above).

7. The original request that you made to cause the authentication process will be retried. Hopefully you've authenticated with sufficient granted authorities to access the protected resource. If you have sufficient access, the request will be successful. Otherwise, you'll receive back an HTTP error code 403, which means "forbidden".

Spring Security has distinct classes responsible for most of the steps described above. The main participants (in the order that they are used) are the ExceptionTranslationFilter, an AuthenticationEntryPoint, an authentication mechanism, and an AuthenticationProvider.

### ExceptionTranslationFilter

ExceptionTranslationFilter is a Spring Security filter that has responsibility for detecting any Spring Security exceptions that are thrown. Such exceptions will generally be thrown by an AbstractSecurityInterceptor, which is the main provider of authorization services. We will discuss AbstractSecurityInterceptor in the next section, but for now we just need to know that it produces Java exceptions and knows nothing about HTTP or how to go about authenticating a principal. Instead the ExceptionTranslationFilter offers this service, with specific responsibility for either returning error code 403 (if the principal has been authenticated and therefore simply lacks sufficient access - as per step seven above), or launching an AuthenticationEntryPoint (if the principal has not been authenticated and therefore we need to go commence step three).

### AuthenticationEntryPoint

The AuthenticationEntryPoint is responsible for step three in the above list. As you can imagine, each web application will have a default authentication strategy (well, this can be configured like nearly everything else in Spring Security, but let's keep it simple for now). Each major authentication system will have its own AuthenticationEntryPoint implementation, which takes actions such as described in step three.

After your browser decides to submit your authentication credentials (either as an HTTP form post or HTTP header) there needs to be something on the server that "collects" these authentication details. By now we're at step six in the above list. In Spring Security we have a special name for the function of collecting authentication details from a user agent (usually a web browser), and that name is "authentication mechanism". After the authentication details are collected from the user agent, an "Authentication request" object is built and then presented to an AuthenticationProvider.

### AuthenticationProvider

The last player in the Spring Security authentication process is an AuthenticationProvider. Quite simply, it is responsible for taking an Authentication request object and deciding whether or not it is valid. The provider will either throw an exception or return a fully populated Authentication object. Remember our good friends, UserDetails and UserDetailsService? If not, head back to the previous section and refresh your memory. Most AuthenticationProviders will ask a UserDetailsService to provide a UserDetails object. As mentioned earlier, most application will provide their own UserDetailsService, although some will be able to use the JDBC or in-memory implementation that ships with Spring Security. The resultant UserDetails object - and particularly the `GrantedAuthority[]`s contained within the UserDetails object - will be used when building the fully populated Authentication object.

After the authentication mechanism receives back the fully-populated Authentication object, it will deem the request valid, put the Authentication into the SecurityContextHolder, and cause the original request to be retried (step seven above). If, on the other hand, the AuthenticationProvider rejected the request, the authentication mechanism will ask the user agent to retry (step two above).

### Setting the SecurityContextHolder Contents Directly

Whilst this describes the typical authentication workflow, the good news is that Spring Security doesn't mind how you put an Authentication inside the SecurityContextHolder. The only critical requirement is that the SecurityContextHolder contains an Authentication that represents a principal before the AbstractSecurityInterceptor needs to authorize a request.

You can (and many users do) write their own filters or MVC controllers to provide interoperability with authentication systems that are not based on Spring Security. For example, you might be using Container-Managed Authentication which makes the current user available from a ThreadLocal or JNDI location. Or you might work for a company that has a legacy proprietary authentication system, which is a corporate "standard" over which you have little control. In such situations it's quite easy to get Spring Security to work, and still provide authorization capabilities. All you need to do is write a filter (or equivalent) that reads the third-party user information from a location, build a Spring Security-specific Authentication object, and put it onto the SecurityContextHolder. It's quite easy to do this, and it is a fully-supported integration approach.

## Secure Objects

Spring Security uses the term "secure object" to refer to any object that can have security (such as an authorization decision) applied to it. The most common examples are method invocations and web requests.

### Security and AOP Advice

If you're familiar with AOP, you'd be aware there are different types of advice available: before, after, throws and around. An around advice is very useful, because an advisor can elect whether or not to proceed with a method invocation, whether or not to modify the response, and whether or not to throw an exception. Spring Security provides an around advice for method invocations as well as web requests. We achieve an around advice for method invocations using Spring's standard AOP support and we achieve an around advice for web requests using a standard Filter.

For those not familiar with AOP, the key point to understand is that Spring Security can help you protect method invocations as well as web requests. Most people are interested in securing method invocations on their services layer. This is because the services layer is where most business logic resides in current-generation J2EE applications (for clarification, the author disapproves of this design and instead advocates properly encapsulated domain objects together with the DTO, assembly, facade and transparent persistence patterns, but as use of anemic domain objects is the present mainstream approach, we'll talk about it here). If you just need to secure method invocations to the services layer, Spring's standard AOP (otherwise known as AOP Alliance) will be adequate. If you need to secure domain objects directly, you will likely find that AspectJ is worth considering.

You can elect to perform method authorization using AspectJ or Spring AOP, or you can elect to perform web request authorization using filters. You can use zero, one, two or three of these approaches together. The mainstream usage is to perform some web request authorization, coupled with some Spring AOP method invocation authorization on the services layer.

### AbstractSecurityInterceptor

Each secure object type supported by Spring Security has its own class, which is a subclass of AbstractSecurityInterceptor. Importantly, by the time the AbstractSecurityInterceptor is called, the SecurityContextHolder will contain a valid Authentication if the principal has been authenticated.

AbstractSecurityInterceptor provides a consistent workflow for handling secure object requests, typically:

1. Look up the "configuration attributes" associated with the present request

2. Submitting the secure object, current Authentication and configuration attributes to the AccessDecisionManager for an authorization decision

3. Optionally change the Authentication under which the invocation takes place

4. Allow the secure object to proceed (assuming access was granted)

5. Call the `AfterInvocationManager` if configured, once the invocation has returned.

#### What are Configuration Attributes?

A "configuration attribute" can be thought of as a String that has special meaning to the classes used by AbstractSecurityInterceptor. They may be simple role names or have more complex meaning, depending on the how sophisticated the AccessDecisionManager implementation is. The AbstractSecurityInterceptor is configured with an ObjectDefinitionSource which it uses to look up the attributes for a secure object. Usually this configuration will be hidden from the user. Configuration attributes will be entered as annotations on secured methods, or as access attributes on secured URLs (using the namespace `<intercept-url>` syntax).

#### RunAsManager

Assuming AccessDecisionManager decides to allow the request, the AbstractSecurityInterceptor will normally just proceed with the request. Having said that, on rare occasions users may want to replace the Authentication inside the SecurityContext with a different Authentication, which is handled by the AccessDecisionManager calling a `RunAsManager`. This might be useful in reasonably unusual situations, such as if a services layer method needs to call a remote system and present a different identity. Because Spring Security automatically propagates security identity from one server to another (assuming you're using a properly-configured RMI or HttpInvoker remoting protocol client), this may be useful.

#### AfterInvocationManager

Following the secure object proceeding and then returning - which may mean a method invocation completing or a filter chain proceeding - the AbstractSecurityInterceptor gets one final chance to handle the invocation. At this stage the AbstractSecurityInterceptor is interested in possibly modifying the return object. We might want this to happen because an authorization decision couldn't be made "on the way in" to a secure object invocation. Being highly pluggable, AbstractSecurityInterceptor will pass control to an `AfterInvocationManager` to actually modify the object if needed. This class can even entirely replace the object, or throw an exception, or not change it in any way.

AbstractSecurityInterceptor and its related objects are shown in .

The key "secure object" model

#### Extending the Secure Object Model

Only developers contemplating an entirely new way of intercepting and authorizing requests would need to use secure objects directly. For example, it would be possible to build a new secure object to secure calls to a messaging system. Anything that requires security and also provides a way of intercepting a call (like the AOP around advice semantics) is capable of being made into a secure object. Having said that, most Spring applications will simply use the three currently supported secure object types (AOP Alliance MethodInvocation, AspectJ `JoinPoint` and web request FilterInvocation) with complete transparency.