replaced Thread initialization with ExecutorService pool for delete

method. This is much faster and produces less blocking when using the
Compressor class which is used by the HTCache. I.e. picture search is
much faster now.

source/net/yacy/kelondro/blob/ArrayStack.java

@@ -41,7 +41,9 @@ import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.ExecutorCompletionService;
 import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
 import java.util.concurrent.Future;
+import java.util.concurrent.FutureTask;
 import java.util.concurrent.LinkedBlockingQueue;
 import java.util.concurrent.RejectedExecutionException;
 import java.util.concurrent.ThreadPoolExecutor;
@@ -839,30 +841,35 @@ public class ArrayStack implements BLOB {
             final blobItem bi = this.blobs.get(0);
             bi.blob.delete(key);
         } else {
-            final Thread[] t = new Thread[this.blobs.size() - 1];
+            @SuppressWarnings("unchecked")
+            final FutureTask<Boolean>[] t = new FutureTask[this.blobs.size() - 1];
             int i = 0;
             for (final blobItem bi: this.blobs) {
                 if (i < t.length) {
                     // run this in a concurrent thread
                     final blobItem bi0 = bi;
-                    t[i] = new Thread() {
+                    t[i] = new FutureTask<Boolean>(new Callable<Boolean>() {
                         @Override
-                        public void run() {
+                        public Boolean call() {
                             try { bi0.blob.delete(key); } catch (final IOException e) {}
+                            return true;
                         }
-                    };
-                    t[i].start();
+                    });
+                    DELETE_EXECUTOR.execute(t[i]);
                 } else {
                     // no additional thread, run in this thread
                     try { bi.blob.delete(key); } catch (final IOException e) {}
                 }
                 i++;
             }
-            for (final Thread s: t) try {s.join();} catch (final InterruptedException e) {}
+            // wait for termination
+            for (final FutureTask<Boolean> s: t) try {s.get();} catch (final InterruptedException e) {} catch (ExecutionException e) {}
         }
         assert mem() <= m : "m = " + m + ", mem() = " + mem();
     }
 
+    private static final ExecutorService DELETE_EXECUTOR = Executors.newFixedThreadPool(128);
+
     /**
      * close the BLOB
      */