Why is web3j java not generating correct return types for my contract

Question

Here is my contract:

pragma solidity ^0.4.17;
contract SignVerify {
    function recoverAddr(bytes32 msgHash, uint8 v, bytes32 r, bytes32 s) returns (address) {
        return ecrecover(msgHash, v, r, s);
    }

    function isSigned(address _addr, bytes32 msgHash, uint8 v, bytes32 r, bytes32 s) returns (bool) {
        return ecrecover(msgHash, v, r, s) == _addr;
    }
}

The corresponding java code generated by web3j (version 3.2.0) contains the following generated method for isSigned.

public RemoteCall<TransactionReceipt> isSigned(String _addr, byte[] msgHash, BigInteger v, byte[] r, byte[] s) {
        Function function = new Function(
                "isSigned", 
                Arrays.<Type>asList(new org.web3j.abi.datatypes.Address(_addr), 
                new org.web3j.abi.datatypes.generated.Bytes32(msgHash), 
                new org.web3j.abi.datatypes.generated.Uint8(v), 
                new org.web3j.abi.datatypes.generated.Bytes32(r), 
                new org.web3j.abi.datatypes.generated.Bytes32(s)), 
                Collections.<TypeReference<?>>emptyList());
        return executeRemoteCallTransaction(function);
    }

Notice the Collections.<TypeReference<?>>emptyList()? that is the output return types. Why is it an empty list?
In the well known Greeter example which returns a string that line becomes Arrays.<TypeReference<?>>asList(new TypeReference<Utf8String>() {}).

The web3j script to generate java code is pretty straight forward:

web3j solidity generate \
        ${dirName}/build/${fileName}.bin \
        ${dirName}/build/${fileName}.abi \
        -p org.web3j.sample.contracts.generated \
        -o ../../java/ > /dev/null

I tried using --javaTypes and --solidtyTypes with no change to generated code. My suspicion now is that web3j only generates return types for constant functions on the contract. Let me know if I can somehow get the return type(the boolean) from the TransactionReceipt returned by the generated java method.

Answer 1

Smart contracts have two main types of functions:

- Transactions: transaction on the Blockchain can change the state of the Blockchain by modifying a smart contract storage for instance. By design, a transaction is asynchronous because, it needs to be broadcasted on the network, picked up by a miner before being added to a block. This process can take a long time, that's why a transaction function can't return a value even if declared in the definition.

- Calls: Function that doesn't change the global state of the blockchain. A call is however synchronous and return a value.

---

In Web3j, in order to distinguish transaction functions from call functions, you need to use the keyword view (or constant for solidity 0.4.x) in your definition.

pragma solidity ^0.4.17;
contract SignVerify {
    function recoverAddr(bytes32 msgHash, uint8 v, bytes32 r, bytes32 s) constant returns (address) {
        return ecrecover(msgHash, v, r, s);
    }

    function isSigned(address _addr, bytes32 msgHash, uint8 v, bytes32 r, bytes32 s) constant returns (bool) {
        return ecrecover(msgHash, v, r, s) == _addr;
    }
}

Given this, Web3j Smart Contract wrapper will know it is a call function that can return a value instead of transaction function.

    public RemoteCall<Boolean> isSigned(String _addr, byte[] msgHash, BigInteger v, byte[] r, byte[] s) {
        final Function function = new Function(FUNC_ISSIGNED, 
                Arrays.<Type>asList(new org.web3j.abi.datatypes.Address(_addr), 
                new org.web3j.abi.datatypes.generated.Bytes32(msgHash), 
                new org.web3j.abi.datatypes.generated.Uint8(v), 
                new org.web3j.abi.datatypes.generated.Bytes32(r), 
                new org.web3j.abi.datatypes.generated.Bytes32(s)), 
                Arrays.<TypeReference<?>>asList(new TypeReference<Bool>() {}));
        return executeRemoteCallSingleValueReturn(function, Boolean.class);
    }

    public RemoteCall<String> recoverAddr(byte[] msgHash, BigInteger v, byte[] r, byte[] s) {
        final Function function = new Function(FUNC_RECOVERADDR, 
                Arrays.<Type>asList(new org.web3j.abi.datatypes.generated.Bytes32(msgHash), 
                new org.web3j.abi.datatypes.generated.Uint8(v), 
                new org.web3j.abi.datatypes.generated.Bytes32(r), 
                new org.web3j.abi.datatypes.generated.Bytes32(s)), 
                Arrays.<TypeReference<?>>asList(new TypeReference<Address>() {}));
        return executeRemoteCallSingleValueReturn(function, String.class);
    }