Add MultiplexingDiscovery.forServices() that builds a suitable set of…

… peer discoveries for

(optional) a set of desired services.

core/src/main/java/org/bitcoinj/net/discovery/MultiplexingDiscovery.java

@@ -18,7 +18,12 @@
 package org.bitcoinj.net.discovery;
 
 import com.google.common.collect.Lists;
+import com.squareup.okhttp.OkHttpClient;
+
 import org.bitcoinj.core.NetworkParameters;
+import org.bitcoinj.core.VersionMessage;
+import org.bitcoinj.net.discovery.HttpDiscovery;
+import org.bitcoinj.net.discovery.DnsDiscovery.DnsSeedDiscovery;
 import org.bitcoinj.utils.*;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -43,6 +48,30 @@ public class MultiplexingDiscovery implements PeerDiscovery {
     protected final NetworkParameters netParams;
     private volatile ExecutorService vThreadPool;
 
+    /**
+     * Builds a suitable set of peer discoveries. Will query them in parallel before producing a merged response.
+     * If specific services are required, DNS is not used as the protocol can't handle it.
+     * @param params Network to use.
+     * @param services Required services as a bitmask, e.g. {@link VersionMessage#NODE_NETWORK}.
+     */
+    public static MultiplexingDiscovery forServices(NetworkParameters params, long services) {
+        List<PeerDiscovery> discoveries = Lists.newArrayList();
+        HttpDiscovery.Details[] httpSeeds = params.getHttpSeeds();
+        if (httpSeeds != null) {
+            OkHttpClient httpClient = new OkHttpClient();
+            for (HttpDiscovery.Details httpSeed : httpSeeds)
+                discoveries.add(new HttpDiscovery(params, httpSeed, httpClient));
+        }
+        // Also use DNS seeds if there is no specific service requirement
+        if (services == 0) {
+            String[] dnsSeeds = params.getDnsSeeds();
+            if (dnsSeeds != null)
+                for (String dnsSeed : dnsSeeds)
+                    discoveries.add(new DnsSeedDiscovery(params, dnsSeed));
+        }
+        return new MultiplexingDiscovery(params, discoveries);
+    }
+
     /**
      * Will query the given seeds in parallel before producing a merged response.
      */