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Customized SumActuator Having a tailored actuator is key to building a java-tron-based customized public chain. This article illustrates how to develop a java-tron-based `SumActuator`.

Actuator module is divided into 4 different methods that are defined in the `Actuator` interface:

  1. `execute`: execute specific actions of transactions, such as state modification, communication between modules, logic execution, etc.

  2. `validate`: define the validation logic of transactions.

  3. `getOwnerAddress`: acquire the address of transaction initiator.

  4. `calcFee`: define the logic for calculating transaction fees.

# Define and register the contract

Currently, contracts supported by java-tron are defined under `src/main/protos/core/contract` directory in protocol module. First creating a `math_contract.proto` file under this directory and declaring `SumContract`. You can also implement any mathematical calculation you want, such as `Subtraction`.

The logic for `SumContract` is the summation of two numerical values:


Meanwhile, register the new contract type in `Transaction.Contract.ContractType` emuneration within the `src/main/protos/core/Tron.proto file`. Important data structures, such as transactions, accounts and blocks, are defined in the `Tron.proto` file:

Then register a function to ensure that gRPC can receive and identify the requests of this contract. Currently, gRPC protocols are all defined in `src/main/protos/api/api.proto`. To add an `InvokeSum` interface in Wallet Service:

At last, recompile the modified proto files. Compiling the java-tron project directly will compile the proto files as well, `protoc` command is also supported.
Currently, java-tron uses protoc v3.4.0. Please keep the same version when compiling by `protoc` command.

After compilation, the corresponding .class under the java_out directory will be updated.
# Implement SumActuator
For now, the default Actuator supported by java-tron is located in `org.tron.core.actuator`. Creating `SumActuator` under this directory:

For simplicity, the above implementation prints the output of SumActuator directly to a log file. If there is any information that need to be stored, consider creating a new chainbase to store the data (guidance on how to create a chainbase will be revealed soon).
As `SumActuator` finished, `invokeSum(MathContract.SumContract req, StreamObserver<Transaction> responseObserver)` function in RpcApiService's sub-class `WalletApi` need to be implemented to receive and process `SumContract`.

# Validate SumActuator
At last, run a test class to validate whether the above steps are correct:

Running SumActuatorTest and the log will print outputs like this: `SumActuator: param1 = 1, param2 = 2, sum = 3`. Here is the output:

At this point, SumActuator is finished. It is a simple case. In real business scenarios, there are much extra work to do, such as wallet-cli supportation or customizing a chainbase for storing data.