How to sign a transaction's inputs?

[Private Z]

I get an unsign transaction's hex.
Like this:  
020000000157714d146d6e04a2693f52db1244d209d41105f1e2bb90a541f3ccbf0861aa6e0100000000ffffffff0118ddf505000000001976a914797313e7510bb9c7c658a1a6e999fe7739eed44688ac00000000

step 1: create an eckey from privatekey
 
DumpedPrivateKey privateKey = 
DumpedPrivateKey.fromBase58(mNetworkParameters, key);
ECKey ECKey = privateKey.getKey();

step 2: crate an unsign transaction.

NetworkParameters networkParameters = TestNet3Params.get();
// hex2bytes is a conversion method that can hex to bytes.
Transaction transaction = new Transaction(networkParameters, 
hex2bytes(signedTx));

Now, i don't know how to sign the transaction？

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