Added fix for recovering from connection timeout issues in filter, as…

… per #279, #197, #144.

core/src/main/java/org/web3j/protocol/core/filters/Filter.java

@@ -9,6 +9,9 @@
 import java.util.concurrent.ScheduledFuture;
 import java.util.concurrent.TimeUnit;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 import org.web3j.protocol.Web3j;
 
 import org.web3j.protocol.core.Request;
@@ -23,6 +26,8 @@
  */
 public abstract class Filter<T> {
 
+    private static final Logger log = LoggerFactory.getLogger(Filter.class);
+
     final Web3j web3j;
     final Callback<T> callback;
 
@@ -43,13 +48,38 @@ public void run(ScheduledExecutorService scheduledExecutorService, long blockTim
             }
 
             filterId = ethFilter.getFilterId();
-
-            scheduledExecutorService.submit(this::getInitialFilterLogs);
-
+            // this runs in the caller thread as if any exceptions are encountered, we shouldn't
+            // proceed with creating the scheduled task below
+            getInitialFilterLogs();
+
+            /*
+            We want the filter to be resilient against client issues. On numerous occasions
+            users have reported socket timeout exceptions when connected over HTTP to Geth and
+            Parity clients. For examples, refer to
+            https://github.com/web3j/web3j/issues/144 and
+            https://github.com/ethereum/go-ethereum/issues/15243.
+
+            Hence we consume errors and log them as errors, allowing our polling for changes to
+            resume. The downside of this approach is that users will not be notified of
+            downstream connection issues. But given the intermittent nature of the connection
+            issues, this seems like a reasonable compromise.
+
+            The alternative approach would be to have another thread that blocks waiting on
+            schedule.get(), catching any Exceptions thrown, and passing them back up to the
+            caller. However, the user would then be required to recreate subscriptions manually
+            which isn't ideal given the aforementioned issues.
+            */
             schedule = scheduledExecutorService.scheduleAtFixedRate(
-                    () -> this.pollFilter(ethFilter),
+                    () -> {
+                        try {
+                            this.pollFilter(ethFilter);
+                        } catch (Throwable e) {
+                            // All exceptions must be caught, otherwise our job terminates without
+                            // any notification
+                            log.error("Error sending request", e);
+                        }
+                    },
                     0, blockTime, TimeUnit.MILLISECONDS);
-
         } catch (IOException e) {
             throwException(e);
         }
@@ -66,6 +96,7 @@ private void getInitialFilterLogs() {
                 ethLog.setResult(Collections.emptyList());
             }
             process(ethLog.getLogs());
+
         } catch (IOException e) {
             throwException(e);
         }
@@ -80,8 +111,9 @@ private void pollFilter(EthFilter ethFilter) {
         }
         if (ethLog.hasError()) {
             throwException(ethFilter.getError());
+        } else {
+            process(ethLog.getLogs());
         }
-        process(ethLog.getLogs());
     }
 
     abstract EthFilter sendRequest() throws IOException;

core/src/main/java/org/web3j/protocol/http/HttpService.java

@@ -18,8 +18,6 @@
 import org.slf4j.LoggerFactory;
 
 import org.web3j.protocol.Service;
-import org.web3j.protocol.core.Request;
-import org.web3j.protocol.core.Response;
 import org.web3j.protocol.exceptions.ClientConnectionException;
 
 /**