

[[concurrency]]
== Concurrency Support

In most environments, Security is stored on a per `Thread` basis.
This means that when work is done on a new `Thread`, the `SecurityContext` is lost.
Spring Security provides some infrastructure to help make this much easier for users.
Spring Security provides low level abstractions for working with Spring Security in multi-threaded environments.
In fact, this is what Spring Security builds on to integration with <<servletapi-start-runnable>> and <<mvc-async>>.

=== DelegatingSecurityContextRunnable

One of the most fundamental building blocks within Spring Security's concurrency support is the `DelegatingSecurityContextRunnable`.
It wraps a delegate `Runnable` in order to initialize the `SecurityContextHolder` with a specified `SecurityContext` for the delegate.
It then invokes the delegate Runnable ensuring to clear the `SecurityContextHolder` afterwards.
The `DelegatingSecurityContextRunnable` looks something like this:

[source,java]
----
public void run() {
try {
	SecurityContextHolder.setContext(securityContext);
	delegate.run();
} finally {
	SecurityContextHolder.clearContext();
}
}
----

While very simple, it makes it seamless to transfer the SecurityContext from one Thread to another.
This is important since, in most cases, the SecurityContextHolder acts on a per Thread basis.
For example, you might have used Spring Security's <<nsa-global-method-security>> support to secure one of your services.
You can now easily transfer the `SecurityContext` of the current `Thread` to the `Thread` that invokes the secured service.
An example of how you might do this can be found below:

[source,java]
----
Runnable originalRunnable = new Runnable() {
public void run() {
	// invoke secured service
}
};

SecurityContext context = SecurityContextHolder.getContext();
DelegatingSecurityContextRunnable wrappedRunnable =
	new DelegatingSecurityContextRunnable(originalRunnable, context);

new Thread(wrappedRunnable).start();
----

The code above performs the following steps:

* Creates a `Runnable` that will be invoking our secured service.
Notice that it is not aware of Spring Security
* Obtains the `SecurityContext` that we wish to use from the `SecurityContextHolder` and initializes the `DelegatingSecurityContextRunnable`
* Use the `DelegatingSecurityContextRunnable` to create a Thread
* Start the Thread we created

Since it is quite common to create a `DelegatingSecurityContextRunnable` with the `SecurityContext` from the `SecurityContextHolder` there is a shortcut constructor for it.
The following code is the same as the code above:


[source,java]
----
Runnable originalRunnable = new Runnable() {
public void run() {
	// invoke secured service
}
};

DelegatingSecurityContextRunnable wrappedRunnable =
	new DelegatingSecurityContextRunnable(originalRunnable);

new Thread(wrappedRunnable).start();
----

The code we have is simple to use, but it still requires knowledge that we are using Spring Security.
In the next section we will take a look at how we can utilize `DelegatingSecurityContextExecutor` to hide the fact that we are using Spring Security.

=== DelegatingSecurityContextExecutor

In the previous section we found that it was easy to use the `DelegatingSecurityContextRunnable`, but it was not ideal since we had to be aware of Spring Security in order to use it.
Let's take a look at how `DelegatingSecurityContextExecutor` can shield our code from any knowledge that we are using Spring Security.

The design of `DelegatingSecurityContextExecutor` is very similar to that of `DelegatingSecurityContextRunnable` except it accepts a delegate `Executor` instead of a delegate `Runnable`.
You can see an example of how it might be used below:


[source,java]
----
SecurityContext context = SecurityContextHolder.createEmptyContext();
Authentication authentication =
	new UsernamePasswordAuthenticationToken("user","doesnotmatter", AuthorityUtils.createAuthorityList("ROLE_USER"));
context.setAuthentication(authentication);

SimpleAsyncTaskExecutor delegateExecutor =
	new SimpleAsyncTaskExecutor();
DelegatingSecurityContextExecutor executor =
	new DelegatingSecurityContextExecutor(delegateExecutor, context);

Runnable originalRunnable = new Runnable() {
public void run() {
	// invoke secured service
}
};

executor.execute(originalRunnable);
----

The code performs the following steps:

* Creates the `SecurityContext` to be used for our `DelegatingSecurityContextExecutor`.
Note that in this example we simply create the `SecurityContext` by hand.
However, it does not matter where or how we get the `SecurityContext` (i.e. we could obtain it from the `SecurityContextHolder` if we wanted).
* Creates a delegateExecutor that is in charge of executing submitted ``Runnable``s
* Finally we create a `DelegatingSecurityContextExecutor` which is in charge of wrapping any Runnable that is passed into the execute method with a `DelegatingSecurityContextRunnable`.
It then passes the wrapped Runnable to the delegateExecutor.
In this instance, the same `SecurityContext` will be used for every Runnable submitted to our `DelegatingSecurityContextExecutor`.
This is nice if we are running background tasks that need to be run by a user with elevated privileges.
* At this point you may be asking yourself "How does this shield my code of any knowledge of Spring Security?" Instead of creating the `SecurityContext` and the `DelegatingSecurityContextExecutor` in our own code, we can inject an already initialized instance of `DelegatingSecurityContextExecutor`.

[source,java]
----
@Autowired
private Executor executor; // becomes an instance of our DelegatingSecurityContextExecutor

public void submitRunnable() {
Runnable originalRunnable = new Runnable() {
	public void run() {
	// invoke secured service
	}
};
executor.execute(originalRunnable);
}
----

Now our code is unaware that the `SecurityContext` is being propagated to the `Thread`, then the `originalRunnable` is executed, and then the `SecurityContextHolder` is cleared out.
In this example, the same user is being used to execute each Thread.
What if we wanted to use the user from `SecurityContextHolder` at the time we invoked `executor.execute(Runnable)` (i.e. the currently logged in user) to process ``originalRunnable``?
This can be done by removing the `SecurityContext` argument from our `DelegatingSecurityContextExecutor` constructor.
For example:


[source,java]
----
SimpleAsyncTaskExecutor delegateExecutor = new SimpleAsyncTaskExecutor();
DelegatingSecurityContextExecutor executor =
	new DelegatingSecurityContextExecutor(delegateExecutor);
----

Now anytime `executor.execute(Runnable)` is executed the `SecurityContext` is first obtained by the `SecurityContextHolder` and then that `SecurityContext` is used to create our `DelegatingSecurityContextRunnable`.
This means that we are executing our `Runnable` with the same user that was used to invoke the `executor.execute(Runnable)` code.

=== Spring Security Concurrency Classes

Refer to the Javadoc for additional integrations with both the Java concurrent APIs and the Spring Task abstractions.
They are quite self-explanatory once you understand the previous code.

* DelegatingSecurityContextCallable
* DelegatingSecurityContextExecutor
* DelegatingSecurityContextExecutorService
* DelegatingSecurityContextRunnable
* DelegatingSecurityContextScheduledExecutorService
* DelegatingSecurityContextSchedulingTaskExecutor
* DelegatingSecurityContextAsyncTaskExecutor
* DelegatingSecurityContextTaskExecutor
