链码可看作是超级账本中的智能合约，通常用来处理网络成员达成共识的业务逻辑，任何对账本数据的查询和修改都需要调用链码实现。超级账本链码本质上是实现接口规约的程序，当前开发语言支持Go语言。
采用Go语言开发链码，需要准备如下开发环境：
所有链码都需要实现Chaincode接口，接口声明如下所示。Peer节点在交易中调用链码接口方法，其中Init方法在链码实例化（instantiate）和升级（upgrade）交易中调用，以便链码执行必要的初始化操作。Invoke方法在调用（invoke）交易中使用，处理交易提案内容，修改或读取账本数据。
// Chaincode interface must be implemented by all chaincodes. The fabric runs
// the transactions by calling these functions as specified.
type Chaincode interface {
// Init is called during Instantiate transaction after the chaincode container
// has been established for the first time, allowing the chaincode to
// initialize its internal data
Init(stub ChaincodeStubInterface) pb.Response
// Invoke is called to update or query the ledger in a proposal transaction.
// Updated state variables are not committed to the ledger until the
// transaction is committed.
Invoke(stub ChaincodeStubInterface) pb.Response
}
表-Chain code接口声明表
链码API中另一个重要接口是Chaincode StubInterface，链码与超级账本的交互都通过该接口实现，如获取调用参数、获取操作用户信息、获取交易信息、操作账本、调用其他链码等。
链码需要包含main方法，并在main方法中调用shim. Start函数启动链码，链码启动中建立与Peer节点的通信连接，实现与Peer的交互。
func (t *SimpleChaincode) Init(stub shim.ChaincodeStubInterface) pb.Response {
var err error
_, args := stub.GetFunctionAndParameters()
if len(args) != 1 {
return shim.Error("Incorrect number of arguments. Expecting 2")
}
if _, err = strconv.Atoi(args[0]); err != nil {
return shim.Error("Expecting integer value for asset holding")
}
if err := stub.PutState(Name, [ ]byte(args[0])); err != nil {
return shim.Error(fmt.Sprintf("fail to put state: %v", err))
}
return shim.Success(nil)
}
func (t *SimpleChaincode) Invoke(stub shim.ChaincodeStubInterface) pb.Response {
function, args := stub.GetFunctionAndParameters()
switch function {
case "invoke":
return t.invoke(stub, args)
case "query":
return t.query(stub, args)
default:
return shim.Error("not support function")
}
}
func (t *SimpleChaincode) invoke(stub shim.ChaincodeStubInterface, args [ ]string) pb.Response {
if len(args) != 1 {
return shim.Error("Incorrect number of arguments. Expecting 1")
}
change, err := strconv.Atoi(args[0])
if err != nil {
return shim.Error("Expecting integer value for asset holding")
}
if state, err := stub.GetState(Name); err != nil {
return shim.Error("fail to read asset state")
} else if value, err := strconv.Atoi(string(state)); err != nil {
return shim.Error("fail to parse asset holding")
} else if err := stub.PutState(Name,[ ]byte(strconv.Itoa(value + change))); err != nil{
return shim.Error("fail to change asset holding")
} else {
return shim.Success(nil)
}
}
func (t *SimpleChaincode) query(stub shim.ChaincodeStubInterface, args [ ]string) pb.Response {
if state, err := stub.GetState(Name); err != nil {
return shim.Error("fail to read asset state")
} else if _, err := strconv.Atoi(string(state)); err != nil {
return shim.Error("fail to parse asset holding")
} else {
return shim.Success(state)
}
}
func main() {
err := shim.Start(new(SimpleChaincode))
if err != nil {
fmt.Printf("Error starting Simple chaincode: %v\n", err)
}
}