io.vertx.core

## Interface Context

-     public interface Context

  The execution context of a `Handler` execution.

  When Vert.x provides an event to a handler or calls the start or stop methods of a `Verticle`, the execution is associated with a `Context`.

  Usually a context is an *event-loop context* and is tied to a specific event loop thread. So executions for that context always occur on that exact same event loop thread.

  In the case of worker verticles and running inline blocking code a worker context will be associated with the execution which will use a thread from the worker thread pool.

  When a handler is set by a thread associated with a specific context, the Vert.x will guarantee that when that handler is executed, that execution will be associated with the same context.

  If a handler is set by a thread not associated with a context (i.e. a non Vert.x thread). Then a new context will be created for that handler.

  In other words, a context is propagated.

  This means that when a verticle is deployed, any handlers it sets will be associated with the same context - the context of the verticle.

  This means (in the case of a standard verticle) that the verticle code will always be executed with the exact same thread, so you don't have to worry about multi-threaded acccess to the verticle state and you can code your application as single threaded.

  This class also allows arbitrary data to be `put(java.lang.Object, java.lang.Object)` and `get(java.lang.Object)` on the context so it can be shared easily amongst different handlers of, for example, a verticle instance.

  This class also provides `runOnContext(io.vertx.core.Handler<java.lang.Void>)` which allows an action to be executed asynchronously using the same context.

  Author:
    Tim Fox

- - ### Method Summary

    All Methods Static Methods Instance Methods Abstract Methods Default Methods
    Modifier and Type | Method and Description
    --- | ---
    `JsonObject` | `config()` If the context is associated with a Verticle deployment, this returns the configuration that was specified when the verticle was deployed.
    `String` | `deploymentID()` If the context is associated with a Verticle deployment, this returns the deployment ID of that deployment.
    `Handler<Throwable>` | `exceptionHandler()`
    `Context` | `exceptionHandler(Handler<Throwable> handler)` Set an exception handler called when the context runs an action throwing an uncaught throwable. When this handler is called, `Vertx.currentContext()` will return this context.
    `default <T> Future<T>` | `executeBlocking(Handler<Promise<T>> blockingCodeHandler)` Same as `executeBlocking(Handler, Handler)` but with an `handler` called when the operation completes
    `<T> Future<T>` | `executeBlocking(Handler<Promise<T>> blockingCodeHandler, boolean ordered)` Same as `executeBlocking(Handler, boolean, Handler)` but with an `handler` called when the operation completes
    `<T> void` | `executeBlocking(Handler<Promise<T>> blockingCodeHandler, boolean ordered, Handler<AsyncResult<T>> resultHandler)` Safely execute some blocking code.
    `default <T> void` | `executeBlocking(Handler<Promise<T>> blockingCodeHandler, Handler<AsyncResult<T>> resultHandler)` Invoke `executeBlocking(Handler, boolean, Handler)` with order = true.
    `<T> T` | `get(Object key)` Get some data from the context.
    `int` | `getInstanceCount()`
    `<T> T` | `getLocal(Object key)` Get some local data from the context.
    `boolean` | `isEventLoopContext()` Is the current context an event loop context?
    `static boolean` | `isOnEventLoopThread()` Is the current thread an event thread?
    `static boolean` | `isOnVertxThread()` Is the current thread a Vert.x thread? That's either a worker thread or an event loop thread
    `static boolean` | `isOnWorkerThread()` Is the current thread a worker thread?
    `boolean` | `isWorkerContext()` Is the current context a worker context?
    `Vertx` | `owner()`
    `default List<String>` | `processArgs()` The process args
    `void` | `put(Object key, Object value)` Put some data in the context.
    `void` | `putLocal(Object key, Object value)` Put some local data in the context.
    `boolean` | `remove(Object key)` Remove some data from the context.
    `boolean` | `removeLocal(Object key)` Remove some local data from the context.
    `void` | `runOnContext(Handler<Void> action)` Run the specified action asynchronously on the same context, some time after the current execution has completed.

- - ### Method Detail

    - #### isOnWorkerThread

          static boolean isOnWorkerThread()

      Is the current thread a worker thread?

      NOTE! This is not always the same as calling `isWorkerContext()`. If you are running blocking code from an event loop context, then this will return true but `isWorkerContext()` will return false.

      Returns:
        true if current thread is a worker thread, false otherwise

    - #### isOnEventLoopThread

          static boolean isOnEventLoopThread()

      Is the current thread an event thread?

      NOTE! This is not always the same as calling `isEventLoopContext()`. If you are running blocking code from an event loop context, then this will return false but `isEventLoopContext()` will return true.

      Returns:
        true if current thread is an event thread, false otherwise

    - #### isOnVertxThread

          static boolean isOnVertxThread()

      Is the current thread a Vert.x thread? That's either a worker thread or an event loop thread
      Returns:
        true if current thread is a Vert.x thread, false otherwise

    - #### runOnContext

          void runOnContext(Handler<Void> action)

      Run the specified action asynchronously on the same context, some time after the current execution has completed.
      Parameters:
        `action` - the action to run

    - #### executeBlocking

          <T> void executeBlocking(Handler<Promise<T>> blockingCodeHandler,
                                   boolean ordered,
                                   Handler<AsyncResult<T>> resultHandler)

      Safely execute some blocking code.

      Executes the blocking code in the handler `blockingCodeHandler` using a thread from the worker pool.

      When the code is complete the handler `resultHandler` will be called with the result on the original context (e.g. on the original event loop of the caller).

      A `Future` instance is passed into `blockingCodeHandler`. When the blocking code successfully completes, the handler should call the `Promise.complete(T)` or `Promise.complete(Object)` method, or the `Promise.fail(java.lang.Throwable)` method if it failed.

      The blocking code should block for a reasonable amount of time (i.e no more than a few seconds). Long blocking operations or polling operations (i.e a thread that spin in a loop polling events in a blocking fashion) are precluded.

      When the blocking operation lasts more than the 10 seconds, a message will be printed on the console by the blocked thread checker.

      Long blocking operations should use a dedicated thread managed by the application, which can interact with verticles using the event-bus or `runOnContext(Handler)`

      Type Parameters:
        `T` - the type of the result
      Parameters:
        `blockingCodeHandler` - handler representing the blocking code to run
        `resultHandler` - handler that will be called when the blocking code is complete
        `ordered` - if true then if executeBlocking is called several times on the same context, the executions for that context will be executed serially, not in parallel. if false then they will be no ordering guarantees

    - #### executeBlocking

          default <T> void executeBlocking(Handler<Promise<T>> blockingCodeHandler,
                                           Handler<AsyncResult<T>> resultHandler)

      Invoke `executeBlocking(Handler, boolean, Handler)` with order = true.
      Type Parameters:
        `T` - the type of the result
      Parameters:
        `blockingCodeHandler` - handler representing the blocking code to run
        `resultHandler` - handler that will be called when the blocking code is complete

    - #### executeBlocking

          <T> Future<T> executeBlocking(Handler<Promise<T>> blockingCodeHandler,
                                        boolean ordered)

      Same as `executeBlocking(Handler, boolean, Handler)` but with an `handler` called when the operation completes

    - #### executeBlocking

          default <T> Future<T> executeBlocking(Handler<Promise<T>> blockingCodeHandler)

      Same as `executeBlocking(Handler, Handler)` but with an `handler` called when the operation completes

    - #### deploymentID

          String deploymentID()

      If the context is associated with a Verticle deployment, this returns the deployment ID of that deployment.
      Returns:
        the deployment ID of the deployment or null if not a Verticle deployment

    - #### config

          JsonObject config()

      If the context is associated with a Verticle deployment, this returns the configuration that was specified when the verticle was deployed.
      Returns:
        the configuration of the deployment or null if not a Verticle deployment

    - #### processArgs

          default List<String> processArgs()

      The process args

    - #### isEventLoopContext

          boolean isEventLoopContext()

      Is the current context an event loop context?

      NOTE! when running blocking code using `Vertx.executeBlocking(Handler, Handler)` from a standard (not worker) verticle, the context will still an event loop context and this `this#isEventLoopContext()` will return true.

      Returns:
        true if false otherwise

    - #### isWorkerContext

          boolean isWorkerContext()

      Is the current context a worker context?

      NOTE! when running blocking code using `Vertx.executeBlocking(Handler, Handler)` from a standard (not worker) verticle, the context will still an event loop context and this `this#isWorkerContext()` will return false.

      Returns:
        true if the current context is a worker context, false otherwise

    - #### get

          <T> T get(Object key)

      Get some data from the context.
      Type Parameters:
        `T` - the type of the data
      Parameters:
        `key` - the key of the data
      Returns:
        the data

    - #### put

          void put(Object key,
                   Object value)

      Put some data in the context.

      This can be used to share data between different handlers that share a context

      Parameters:
        `key` - the key of the data
        `value` - the data

    - #### remove

          boolean remove(Object key)

      Remove some data from the context.
      Parameters:
        `key` - the key to remove
      Returns:
        true if removed successfully, false otherwise

    - #### getLocal

          <T> T getLocal(Object key)

      Get some local data from the context.
      Type Parameters:
        `T` - the type of the data
      Parameters:
        `key` - the key of the data
      Returns:
        the data

    - #### putLocal

          void putLocal(Object key,
                        Object value)

      Put some local data in the context.

      This can be used to share data between different handlers that share a context

      Parameters:
        `key` - the key of the data
        `value` - the data

    - #### removeLocal

          boolean removeLocal(Object key)

      Remove some local data from the context.
      Parameters:
        `key` - the key to remove
      Returns:
        true if removed successfully, false otherwise

    - #### owner

          Vertx owner()

      Returns:
        The Vertx instance that created the context

    - #### getInstanceCount

          int getInstanceCount()

      Returns:
        the number of instances of the verticle that were deployed in the deployment (if any) related to this context

    - #### exceptionHandler

          Context exceptionHandler(Handler<Throwable> handler)

      Set an exception handler called when the context runs an action throwing an uncaught throwable.

      When this handler is called, `Vertx.currentContext()` will return this context.
      Parameters:
        `handler` - the exception handler
      Returns:
        a reference to this, so the API can be used fluently

    - #### exceptionHandler

          Handler<Throwable> exceptionHandler()

      Returns:
        the current exception handler of this context

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