add warm up trigger (#183)

* add warm up trigger

* add back name field

Author: kaibocai

src/main/java/com/microsoft/azure/functions/annotation/WarmupTrigger.java

@@ -0,0 +1,50 @@
+package com.microsoft.azure.functions.annotation;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+
+/**
+ * <p> The warmup trigger lets you define a function that's run when a new instance of your function app is started.
+ * You can use a warmup trigger to pre-load custom dependencies during the pre-warming process so your functions are
+ * ready to start processing requests immediately. Some actions for a warmup trigger might include opening connections,
+ * loading dependencies, or running any other custom logic before your app begins receiving traffic.
+ * The parameter type should be set as {@link java.lang.Object}</p>
+ *
+ *
+ * <p>The following example shows a Java function that logs the message body of the event hub trigger:</p>
+ *
+ * <pre>{@literal @}FunctionName("Warmup")
+ * public void warmup(
+ *    {@literal @}WarmupTrigger Object warmupContext,
+ *     final ExecutionContext context
+ * ) {
+ *     context.getLogger().info("Function App instance is warm up");
+ * }</pre>
+ *
+ * @since 2.0.2
+ */
+@Retention(RetentionPolicy.RUNTIME)
+@Target(ElementType.PARAMETER)
+@CustomBinding(direction = "in", name = "warmupContext", type = "warmupTrigger")
+public @interface WarmupTrigger {
+    /**
+     * The variable name used in function code for the request or request body.
+     *
+     * @return The variable name used in function code for the request or request body.
+     */
+    String name() default "warmupContext";
+
+    /**
+     * <p>Defines how Functions runtime should treat the parameter value. Possible values are:</p>
+     * <ul>
+     *     <li>"": get the value as a string, and try to deserialize to actual parameter type like POJO</li>
+     *     <li>string: always get the value as a string</li>
+     *     <li>binary: get the value as a binary data, and try to deserialize to actual parameter type byte[]</li>
+     * </ul>
+     * @return The dataType which will be used by the Functions runtime.
+     */
+    String dataType() default "";
+}