Update Healthcheck and add javadoc (#9542)

* Update Healthcheck and add javadoc

* sebd requests; improve javadoc

* add more javadoc

languagetool-core/src/main/java/org/languagetool/Language.java

@@ -536,6 +536,11 @@ public void setPostDisambiguationChunker(Chunker chunker) {
   /**
    * Create a shared instance of JLanguageTool to use in rules for further processing
    * Instances are shared by Language
+   * As this is a shared instance, do not modify (add or remove) any rules or filters.
+   * The alternative to disabling/enabling rules is to select the desired rules from getAllActiveRules(), and run them separately with rule.match(analizedSentence).
+   *
+   * Do not call this in a static block or to initialize a static JLanguageTool field in rules or filters classes, this could lead to a deadlock during initialization.
+   *
    * @since 6.1
    * @return a shared JLanguageTool instance for this language
    */

languagetool-server/src/main/java/org/languagetool/server/LanguageToolHttpHandler.java

@@ -123,7 +123,8 @@ public void handle(HttpExchange httpExchange) throws IOException {
         // healthcheck should come before other limit checks (requests per time etc.), to be sure it works: 
         String pathWithoutVersion = path.substring("/v2/".length());
         if (pathWithoutVersion.equals("healthcheck")) {
-          if (workQueueFull(httpExchange, parameters, "Healthcheck failed: There are currently too many parallel requests.")) {
+          String message = "Healthcheck failed: There are currently too many parallel requests.";
+          if (workQueueFull(httpExchange, parameters, message) || textCheckerQueueFull(httpExchange, message)) {
             ServerMetricsCollector.getInstance().logFailedHealthcheck();
             return;
           } else {
@@ -323,6 +324,14 @@ private boolean workQueueFull(HttpExchange httpExchange, Map<String, String> par
     return false;
   }
 
+  private boolean textCheckerQueueFull(HttpExchange httpExchange, String response) throws IOException {
+    if(textCheckerV2.checkerQueueAlmostFull()) {
+      sendError(httpExchange, HTTP_UNAVAILABLE, "Error: " + response);
+      return true;
+    }
+    return false;
+  }
+
   @NotNull
   private String getTextOrDataSizeMessage(Map<String, String> parameters) {
     String text = parameters.get("text");

languagetool-server/src/main/java/org/languagetool/server/TextChecker.java

@@ -757,6 +757,19 @@ void checkText(AnnotatedText aText, HttpExchange httpExchange, Map<String, Strin
 
   }
 
+  public boolean checkerQueueAlmostFull() {
+    if (this.executorService instanceof ThreadPoolExecutor) {
+      ThreadPoolExecutor threadPoolExecutor = (ThreadPoolExecutor) this.executorService;
+      int maxQueueSize = config.getTextCheckerQueueSize();
+      int queuesize = threadPoolExecutor.getQueue().size();
+      if (queuesize > maxQueueSize/2) { //should not happen in normal cases (workQueue.size() == config.getTextCheckerQueueSize())
+        log.warn("TextChecker queue is almost full requests in queue: {} active request: {}", queuesize, threadPoolExecutor.getActiveCount());
+        return true;
+      }
+    }
+    return false;
+  }
+
   @NotNull
   private Map<String, Integer> getRuleMatchCount(List<CheckResults> res) {
     Map<String, Integer> ruleMatchCount = new HashMap<>();