How to handle networking/peer APIs

This article applies to the code in git master only


The bitcoinj networking APIs have a few options targeted at different use-cases - you can spin up individual Peers and manage them yourself or bring up a PeerGroup to let it manage them, you can use one-off sockets or socket managers, and you can use blocking sockets or NIO/non-blocking sockets. This page attempts to explain the tradeoffs and use-cases for each choice, as well as provide some basic examples to doing more advanced networking.

The bitcoinj networking API is built up in a series of layers. On the bottom are simple wrapper classes that provide an API to open new connections using blocking sockets or java NIO (asynchronous select()-based sockets). On top of those sit various parsers that parse the network traffic into messages (ie into the Bitcoin messages). On top of those are Peer objects, which handle message handling (exchanging initial version handshake, downloading blocks, etc) for each individual remote peer and provide a simple event listener interface. Finally a PeerGroup can be layered on top to keep track of peers, ensuring there are always enough connections to the network and keeping track of network sync progress.

The simple case

In the case that you simply want a connection to the P2P network (ie in the vast majority of cases), all you need to do is instantiate a PeerGroup and connect a few objects:

PeerGroup peerGroup = new PeerGroup(networkParameters, blockChain);
peerGroup.setUserAgent("Sample App", 1.0);
peerGroup.addPeerDiscovery(new DnsDiscovery(networkParameters));

After this code completes you’ve connected to some peers and fully downloaded the blockchain up to the latest block, filling in missing wallet transactions as it goes. peerGroup.startAndWait() and peerGroup.downloadBlockChain() can be replaced with asynchronous versions peerGroup.start() followed by peerGroup.startBlockchainDownload(listener) when the future returned by start() completes.


If you wish to connect to the network using a SOCKS proxy, you must use blocking sockets instead of nio ones. This makes network slightly less efficient, but it should not be noticeable for anything short of very heavy workloads. Then you simply set the Java system properties for a SOCKS proxy (as below) and connections will automatically flow over the proxy.

System.setProperty("socksProxyHost", "");
System.setProperty("socksProxyPort", "9050");
peerGroup = new PeerGroup(params, chain, new BlockingClientManager());

Using the lower level API’s

You can build a Peer at a lower level, controlling the socket to be used, using code like this:

Peer peer = new Peer(params, bitcoin.chain(), new PeerAddress(InetAddress.getLocalHost(), 8333), "test app", "0.1") {
    public void connectionOpened() {
        System.out.println("TCP connect done");
peer.addEventListener(new AbstractPeerEventListener() {
    public void onPeerConnected(Peer peer, int peerCount) {
        System.out.println("Version handshake done");
new BlockingClient(address, peer, 10000, SocketFactory.getDefault(), null);

Of course you would provide your own SocketFactory instead of using the default.

If you want access to a raw stream of messages with no higher level logic, subclass PeerSocketHandler and override the processMessage abstract method. This class implements StreamParser which breaks raw byte streams into the right subclass of Message for you, and then lets you handle those messages as you see fit. Create instances of your new object and pass them to an implementation of ClientConnectionManager, typically either BlockingClientManager or NioClientManager to use epoll/select based async IO.