
[[el-access]]
= Expression-Based Access Control
Spring Security 3.0 introduced the ability to use Spring Expression Language (SpEL) expressions as an authorization mechanism in addition to the existing configuration attributes and access-decision voters.
Expression-based access control is built on the same architecture but lets complicated Boolean logic be encapsulated in a single expression.


== Overview
Spring Security uses SpEL for expression support and you should look at how that works if you are interested in understanding the topic in more depth.
Expressions are evaluated with a "`root object`" as part of the evaluation context.
Spring Security uses specific classes for web and method security as the root object to provide built-in expressions and access to values, such as the current principal.

[[el-common-built-in]]
=== Common Built-In Expressions
The base class for expression root objects is `SecurityExpressionRoot`.
This provides some common expressions that are available in both web and method security:

[[common-expressions]]
.Common built-in expressions
|===
| Expression | Description

| `hasRole(String role)`
| Returns `true` if the current principal has the specified role.

Example: `hasRole('admin')`

By default, if the supplied role does not start with `ROLE_`, it is added.
You can customize this behavior by modifying the `defaultRolePrefix` on `DefaultWebSecurityExpressionHandler`.

| `hasAnyRole(String... roles)`
| Returns `true` if the current principal has any of the supplied roles (given as a comma-separated list of strings).

Example: `hasAnyRole('admin', 'user')`.

By default, if the supplied role does not start with `ROLE_`, it is added.
You can customize this behavior by modifying the `defaultRolePrefix` on `DefaultWebSecurityExpressionHandler`.

| `hasAuthority(String authority)`
| Returns `true` if the current principal has the specified authority.

Example: `hasAuthority('read')`

| `hasAnyAuthority(String... authorities)`
| Returns `true` if the current principal has any of the supplied authorities (given as a comma-separated list of strings).

Example: `hasAnyAuthority('read', 'write')`.

| `principal`
| Allows direct access to the principal object that represents the current user.

| `authentication`
| Allows direct access to the current `Authentication` object obtained from the `SecurityContext`.

| `permitAll`
| Always evaluates to `true`.

| `denyAll`
| Always evaluates to `false`.

| `isAnonymous()`
| Returns `true` if the current principal is an anonymous user.

| `isRememberMe()`
| Returns `true` if the current principal is a remember-me user.

| `isAuthenticated()`
| Returns `true` if the user is not anonymous.

| `isFullyAuthenticated()`
| Returns `true` if the user is not an anonymous and is not a remember-me user.

| `hasPermission(Object target, Object permission)`
| Returns `true` if the user has access to the provided target for the given permission.
Example, `hasPermission(domainObject, 'read')`.

| `hasPermission(Object targetId, String targetType, Object permission)`
| Returns `true` if the user has access to the provided target for the given permission.
Example, `hasPermission(1, 'com.example.domain.Message', 'read')`.
|===



[[el-access-web]]
== Web Security Expressions
To use expressions to secure individual URLs, you first need to set the `use-expressions` attribute in the `<http>` element to `true`.
Spring Security then expects the `access` attributes of the `<intercept-url>` elements to contain SpEL expressions.
Each expression should evaluate to a Boolean, defining whether access should be allowed or not.
The following listing shows an example:

====
[source,xml]
----
<http>
	<intercept-url pattern="/admin*"
		access="hasRole('admin') and hasIpAddress('192.168.1.0/24')"/>
	...
</http>
----
====

Here, we have defined that the `admin` area of an application (defined by the URL pattern) should be available only to users who have the granted authority (`admin`) and whose IP address matches a local subnet.
We have already seen the built-in `hasRole` expression in the previous section.
The `hasIpAddress` expression is an additional built-in expression that is specific to web security.
It is defined by the `WebSecurityExpressionRoot` class, an instance of which is used as the expression root object when evaluating web-access expressions.
This object also directly exposed the `HttpServletRequest` object under the name `request` so that you can invoke the request directly in an expression.
If expressions are being used, a `WebExpressionVoter` is added to the `AccessDecisionManager` that is used by the namespace.
So, if you do not use the namespace and want to use expressions, you have to add one of these to your configuration.

[[el-access-web-beans]]
=== Referring to Beans in Web Security Expressions

If you wish to extend the expressions that are available, you can easily refer to any Spring Bean you expose.
For example, you could use the following, assuming you have a Bean with the name of `webSecurity` that contains the following method signature:

====
.Java
[source,java,role="primary"]
----
public class WebSecurity {
		public boolean check(Authentication authentication, HttpServletRequest request) {
				...
		}
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
class WebSecurity {
    fun check(authentication: Authentication?, request: HttpServletRequest?): Boolean {
        // ...
    }
}
----
====

You could then refer to the method as follows:

.Refer to method
====
.Java
[source,java,role="primary"]
----
http
    .authorizeHttpRequests(authorize -> authorize
        .antMatchers("/user/**").access("@webSecurity.check(authentication,request)")
        ...
    )
----

.XML
[source,xml,role="secondary"]
----
<http>
	<intercept-url pattern="/user/**"
		access="@webSecurity.check(authentication,request)"/>
	...
</http>
----

.Kotlin
[source,kotlin,role="secondary"]
----
http {
    authorizeRequests {
        authorize("/user/**", "@webSecurity.check(authentication,request)")
    }
}
----
====

[[el-access-web-path-variables]]
=== Path Variables in Web Security Expressions

At times, it is nice to be able to refer to path variables within a URL.
For example, consider a RESTful application that looks up a user by ID from a URL path in a format of `+/user/{userId}+`.

You can easily refer to the path variable by placing it in the pattern.
For example, you could use the following if you had a Bean with the name of `webSecurity` that contains the following method signature:

====
.Java
[source,java,role="primary"]
----
public class WebSecurity {
		public boolean checkUserId(Authentication authentication, int id) {
				...
		}
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
class WebSecurity {
    fun checkUserId(authentication: Authentication?, id: Int): Boolean {
        // ...
    }
}
----
====

You could then refer to the method as follows:

.Path Variables
====
.Java
[source,java,role="primary",attrs="-attributes"]
----
http
	.authorizeHttpRequests(authorize -> authorize
		.antMatchers("/user/{userId}/**").access("@webSecurity.checkUserId(authentication,#userId)")
		...
	);
----

.XML
[source,xml,role="secondary",attrs="-attributes"]
----
<http>
	<intercept-url pattern="/user/{userId}/**"
		access="@webSecurity.checkUserId(authentication,#userId)"/>
	...
</http>
----

.Kotlin
[source,kotlin,role="secondary",attrs="-attributes"]
----
http {
    authorizeRequests {
        authorize("/user/{userId}/**", "@webSecurity.checkUserId(authentication,#userId)")
    }
}
----
====

In this configuration, URLs that match would pass in the path variable (and convert it) into the `checkUserId` method.
For example, if the URL were `/user/123/resource`, the ID passed in would be `123`.

== Method Security Expressions
Method security is a bit more complicated than a simple allow or deny rule.
Spring Security 3.0 introduced some new annotations to allow comprehensive support for the use of expressions.

[[el-pre-post-annotations]]
=== @Pre and @Post Annotations
There are four annotations that support expression attributes to allow pre and post-invocation authorization checks and also to support filtering of submitted collection arguments or return values.
They are `@PreAuthorize`, `@PreFilter`, `@PostAuthorize`, and `@PostFilter`.
Their use is enabled through the `global-method-security` namespace element:

====
[source,xml]
----
<global-method-security pre-post-annotations="enabled"/>
----
====

==== Access Control using @PreAuthorize and @PostAuthorize
The most obviously useful annotation is `@PreAuthorize`, which decides whether a method can actually be invoked or not.
The following example (from the {gh-samples-url}/servlet/xml/java/contacts["Contacts" sample application]) uses the `@PreAuthorize` annotation:

====
.Java
[source,java,role="primary"]
----
@PreAuthorize("hasRole('USER')")
public void create(Contact contact);
----

.Kotlin
[source,kotlin,role="secondary"]
----
@PreAuthorize("hasRole('USER')")
fun create(contact: Contact?)
----
====

This means that access is allowed only for users with the `ROLE_USER` role.
Obviously, the same thing could easily be achieved by using a traditional configuration and a simple configuration attribute for the required role.
However, consider the following example:

====
.Java
[source,java,role="primary"]
----
@PreAuthorize("hasPermission(#contact, 'admin')")
public void deletePermission(Contact contact, Sid recipient, Permission permission);
----

.Kotlin
[source,kotlin,role="secondary"]
----
@PreAuthorize("hasPermission(#contact, 'admin')")
fun deletePermission(contact: Contact?, recipient: Sid?, permission: Permission?)
----
====

Here, we actually use a method argument as part of the expression to decide whether the current user has the `admin` permission for the given contact.
The built-in `hasPermission()` expression is linked into the Spring Security ACL module through the application context, as we <<el-permission-evaluator,see later in this section>>.
You can access any of the method arguments by name as expression variables.

Spring Security can resolve the method arguments in a number of ways.
Spring Security uses `DefaultSecurityParameterNameDiscoverer` to discover the parameter names.
By default, the following options are tried for a method.

* If Spring Security's `@P` annotation is present on a single argument to the method, the value is used.
This is useful for interfaces compiled with a JDK prior to JDK 8 (which do not contain any information about the parameter names).
The following example uses the `@P` annotation:

+

====
.Java
[source,java,role="primary"]
----
import org.springframework.security.access.method.P;

...

@PreAuthorize("#c.name == authentication.name")
public void doSomething(@P("c") Contact contact);
----

.Kotlin
[source,kotlin,role="secondary"]
----
import org.springframework.security.access.method.P

...

@PreAuthorize("#c.name == authentication.name")
fun doSomething(@P("c") contact: Contact?)
----
====

+

Behind the scenes, this is implemented by using `AnnotationParameterNameDiscoverer`, which you can customize to support the value attribute of any specified annotation.

* If Spring Data's `@Param` annotation is present on at least one parameter for the method, the value is used.
This is useful for interfaces compiled with a JDK prior to JDK 8 which do not contain any information about the parameter names.
The following example uses the `@Param` annotation:
+
====
.Java
[source,java,role="primary"]
----
import org.springframework.data.repository.query.Param;

...

@PreAuthorize("#n == authentication.name")
Contact findContactByName(@Param("n") String name);
----

.Kotlin
[source,kotlin,role="secondary"]
----
import org.springframework.data.repository.query.Param

...

@PreAuthorize("#n == authentication.name")
fun findContactByName(@Param("n") name: String?): Contact?
----
====
+

Behind the scenes, this is implemented by using `AnnotationParameterNameDiscoverer`, which you can customize to support the value attribute of any specified annotation.

* If JDK 8 was used to compile the source with the `-parameters` argument and Spring 4+ is being used, the standard JDK reflection API is used to discover the parameter names.
This works on both classes and interfaces.

* Finally, if the code was compiled with the debug symbols, the parameter names are discovered by using the debug symbols.
This does not work for interfaces, since they do not have debug information about the parameter names.
For interfaces, annotations or the JDK 8 approach must be used.

.[[el-pre-post-annotations-spel]]
Any SpEL functionality is available within the expression, so you can also access properties on the arguments.
For example, if you wanted a particular method to allow access only to a user whose username matched that of the contact, you could write

====
.Java
[source,java,role="primary"]
----
@PreAuthorize("#contact.name == authentication.name")
public void doSomething(Contact contact);
----

.Kotlin
[source,kotlin,role="secondary"]
----
@PreAuthorize("#contact.name == authentication.name")
fun doSomething(contact: Contact?)
----
====

Here we are accessing another built-in expression, `authentication`, which is the `Authentication` stored in the security context.
You can also access its "principal" property directly, using the expression `principal`.
The value will often be a `UserDetails` instance, so you might use an expression like `principal.username` or `principal.enabled`.

.[[el-pre-post-annotations-post]]
Here, we access another built-in expression, `authentication`, which is the `Authentication` stored in the security context.
You can also access its `principal` property directly, by using the `principal` expression.
The value is often a `UserDetails` instance, so you might use an expression such as `principal.username` or `principal.enabled`.

==== Filtering using @PreFilter and @PostFilter
Spring Security supports filtering of collections, arrays, maps, and streams by using expressions.
This is most commonly performed on the return value of a method.
The following example uses `@PostFilter`:

====
.Java
[source,java,role="primary"]
----
@PreAuthorize("hasRole('USER')")
@PostFilter("hasPermission(filterObject, 'read') or hasPermission(filterObject, 'admin')")
public List<Contact> getAll();
----

.Kotlin
[source,kotlin,role="secondary"]
----
@PreAuthorize("hasRole('USER')")
@PostFilter("hasPermission(filterObject, 'read') or hasPermission(filterObject, 'admin')")
fun getAll(): List<Contact?>
----
====

When using the `@PostFilter` annotation, Spring Security iterates through the returned collection or map and removes any elements for which the supplied expression is false.
For an array, a new array instance that contains filtered elements is returned.
`filterObject` refers to the current object in the collection.
When a map is used, it refers to the current `Map.Entry` object, which lets you use `filterObject.key` or `filterObject.value` in the expression.
You can also filter before the method call by using `@PreFilter`, though this is a less common requirement.
The syntax is the same. However, if there is more than one argument that is a collection type, you have to select one by name using the `filterTarget` property of this annotation.

Note that filtering is obviously not a substitute for tuning your data retrieval queries.
If you are filtering large collections and removing many of the entries, this is likely to be inefficient.


[[el-method-built-in]]
=== Built-In Expressions
There are some built-in expressions that are specific to method security, which we have already seen in use earlier.
The `filterTarget` and `returnValue` values are simple enough, but the use of the `hasPermission()` expression warrants a closer look.


[[el-permission-evaluator]]
==== The PermissionEvaluator interface
`hasPermission()` expressions are delegated to an instance of `PermissionEvaluator`.
It is intended to bridge between the expression system and Spring Security's ACL system, letting you specify authorization constraints on domain objects, based on abstract permissions.
It has no explicit dependencies on the ACL module, so you could swap that out for an alternative implementation if required.
The interface has two methods:

====
[source,java]
----
boolean hasPermission(Authentication authentication, Object targetDomainObject,
							Object permission);

boolean hasPermission(Authentication authentication, Serializable targetId,
							String targetType, Object permission);
----
====

These methods map directly to the available versions of the expression, with the exception that the first argument (the `Authentication` object) is not supplied.
The first is used in situations where the domain object, to which access is being controlled, is already loaded.
Then the expression returns `true` if the current user has the given permission for that object.
The second version is used in cases where the object is not loaded but its identifier is known.
An abstract "`type`" specifier for the domain object is also required, letting the correct ACL permissions be loaded.
This has traditionally been the Java class of the object but does not have to be, as long as it is consistent with how the permissions are loaded.

To use `hasPermission()` expressions, you have to explicitly configure a `PermissionEvaluator` in your application context.
The following example shows how to do so:

====
[source,xml]
----
<security:global-method-security pre-post-annotations="enabled">
<security:expression-handler ref="expressionHandler"/>
</security:global-method-security>

<bean id="expressionHandler" class=
"org.springframework.security.access.expression.method.DefaultMethodSecurityExpressionHandler">
	<property name="permissionEvaluator" ref="myPermissionEvaluator"/>
</bean>
----
====

Where `myPermissionEvaluator` is the bean which implements `PermissionEvaluator`.
Usually, this is the implementation from the ACL module, which is called `AclPermissionEvaluator`.
See the {gh-samples-url}/servlet/xml/java/contacts[`Contacts`] sample application configuration for more details.

==== Method Security Meta Annotations

You can make use of meta annotations for method security to make your code more readable.
This is especially convenient if you find that you repeat the same complex expression throughout your code base.
For example, consider the following:

====
[source,java]
----
@PreAuthorize("#contact.name == authentication.name")
----
====

Instead of repeating this everywhere, you can create a meta annotation:

====
.Java
[source,java,role="primary"]
----
@Retention(RetentionPolicy.RUNTIME)
@PreAuthorize("#contact.name == authentication.name")
public @interface ContactPermission {}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Retention(AnnotationRetention.RUNTIME)
@PreAuthorize("#contact.name == authentication.name")
annotation class ContactPermission
----
====

You can use meta annotations for any of the Spring Security method security annotations.
To remain compliant with the specification, JSR-250 annotations do not support meta annotations.

