feat: add PortOpen example

examples/src/main/java/org/bitcoinj/examples/PortOpen.java

@@ -0,0 +1,75 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.bitcoinj.examples;
+
+import org.bitcoinj.core.NetworkParameters;
+import org.bitcoinj.net.discovery.PeerDiscoveryException;
+import org.bitcoinj.net.discovery.SeedPeers;
+import org.bitcoinj.params.MainNetParams;
+
+import java.io.IOException;
+import java.net.InetSocketAddress;
+import java.net.Socket;
+import java.util.concurrent.TimeUnit;
+
+/**
+ * Determine which of the seed nodes have open ports on mainnet
+ */
+public class PortOpen {
+    private static boolean available(String address, int port) {
+        try (Socket ignored = new Socket(address, port)) {
+            return false;
+        } catch (IOException ignored) {
+            return true;
+        }
+    }
+
+    // https://www.geekality.net/2013/04/30/java-simple-check-to-see-if-a-server-is-listening-on-a-port/
+    public static boolean serverListening(String host, int port) {
+        Socket s = null;
+        try {
+            s = new Socket(host, port);
+            return true;
+        } catch (Exception e) {
+            return false;
+        } finally {
+            if (s != null)
+                try {
+                    s.close();
+                } catch (Exception ignored) {
+                    // ignore this exception
+                }
+        }
+    }
+
+    public static void main(String[] args) {
+        NetworkParameters params = MainNetParams.get();
+
+        SeedPeers seedPeers = new SeedPeers(params);
+        try {
+            int index = 0;
+            int notAvailable = 0;
+            for (InetSocketAddress address : seedPeers.getPeers(0, 10, TimeUnit.SECONDS)) {
+                boolean available = serverListening(address.getAddress().getHostAddress(), params.getPort());
+                System.out.println(address.getAddress() + " is " + (available ? "available" : "not available"));
+                index++;
+                notAvailable += available ? 0 : 1;
+            }
+            System.out.println((notAvailable * 100 / index) + "% are not available");
+        } catch (PeerDiscoveryException x) {
+            System.out.println("Error: " + x.getMessage());
+        }
+    }
+}