Blockchain.info has a PushTX API.
How do I create & sign a raw transaction in Java for use in this API? Should I use bitcoinj?
Are there any code samples?
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Mike Hearn's WalletTool (included in the bitcoinj library) is a pretty good starting point.
If you have a look at the 'send' method in WalletTool you can see that it:
1) Creates a SendRequest
2) Decrypts the private key if necessary
3) Completes the tx to get the inputs and find the fee
4) Signs the tx
5) Starts up a PeerGroup
6) Broadcasts it
For an offline transaction you can do 1 to 4, serialize the tx and then call the blockchain pushtx.
There is a method on Transaction called bitcoinSerialize to convert it to the bytes that need transmitting. I think you just have to convert the bytes to hex for the blockchain pushtx but you would have to try it out.
Check out https://github.com/ValleZ/Paper-Wallet - it is android lite project what does this task. Start with createTransaction method in https://github.com/ValleZ/Paper-Wallet/blob/master/app/src/main/java/ru/valle/btc/BTCUtils.java