jdk1.8新特性之方法引用

　　方法引用其实就是方法调用，符号是两个冒号::来表示，左边是对象或类，右边是方法。它其实就是lambda表达式的进一步简化。如果不使用lambda表达式，那么也就没必要用方法引用了。啥是lambda，参见jdk1.8新特性之lambda表达式。看实际例子：

　　先看函数式接口：

@FunctionalInterface
public interface CompositeServiceMethodInvoker<M extends Message, R extends Message>
{

Logger LOGGER = LoggerFactory.getLogger(CompositeServiceMethodInvoker.class);

ApiResult<M> invoke(InvokeContext ic, R r);

default M getCompositeResponse(R request)
throws PortalException
{
return getCompositeResponse(GetSpringContext.getInvokeContext(), request);
}

default M getCompositeResponse(InvokeContext invokeContext, R request)
throws PortalException
{
if (LOGGER.isDebugEnabled())
{
LOGGER.debug(
"Enter CompositeServiceEngine.getCompositeResponse(), identityId:{}, requestClassName:{}, request:{}",
CommonHttpUtil.getIdentity(),
request.getClass().getName(),
JsonFormatUtil.printToString(request));
}

ApiResult<M> apiResult = invoke(invokeContext, request);

if (Util.isEmpty(apiResult))
{
LOGGER.error(
" CompositeServiceEngine.getCompositeResponse(), Call microservice error, return null, identityId:{}," +
" requestClassName:{}, request:{}",
CommonHttpUtil.getIdentity(),
request.getClass().getName(),
JsonFormatUtil.printToString(request));
throw new PortalException(MSResultCode.MICROSERVICE_RETURN_NULL,
(" CompositeServiceEngine.getCompositeResponse(), Call microservice error, return null, " +
"requestClassName:")
.concat(request.getClass().getName()));
}

int code = apiResult.getCode();
if (!apiResult.isSuccess())
{
LOGGER.error(
"Call CompositeServiceEngine.getCompositeResponse() error, identityId:{}, requestClassName:{}, " +
"request:{}, return code:{}",
CommonHttpUtil.getIdentity(),
request.getClass().getName(),
JsonFormatUtil.printToString(request),
code);
throw new PortalException(code,
"Call CompositeServiceEngine.getCompositeResponse() error, requestClassName:".concat(request.getClass()
.getName()));
}
else
{
M response = apiResult.getData();

if (Util.isEmpty(response))
{
LOGGER.error(
"Call CompositeServiceEngine.getCompositeResponse() error,return null, identityId:{}, " +
"requestClassName:{}, request:{}, return code:{}",
CommonHttpUtil.getIdentity(),
request.getClass().getName(),
JsonFormatUtil.printToString(request),
code);
throw new PortalException(code,
"Call CompositeServiceEngine.getCompositeResponse() error, return null, requestClassName:".concat(
request.getClass().getName()));
}

if (LOGGER.isDebugEnabled())
{
LOGGER.debug(
"Exit CompositeServiceEngine.getCompositeResponse(), identityId:{}, requestClasssName:{}, " +
"request:{}, result:{}",
CommonHttpUtil.getIdentity(),
request.getClass().getName(),
JsonFormatUtil.printToString(request),
JsonFormatUtil.printToString(response));
}
return response;
}
}

default String getCompositeResponseCode(R request)
throws PortalException
{
if (LOGGER.isDebugEnabled())
{
LOGGER.debug(
"Enter CompositeServiceEngine.getCompositeResponse() , identityId:{}, requestClassName:{}, request:{}",
CommonHttpUtil.getIdentity(),
request.getClass().getName(),
JsonFormatUtil.printToString(request));
}

ApiResult<M> apiResult = invoke(GetSpringContext.getInvokeContext(), request);

if (Util.isEmpty(apiResult))
{
LOGGER.error(
" CompositeServiceEngine.getCompositeResponse(), Call microservice error, return null,  " +
"identityId:{}, requestClassName:{}, request:{}",
CommonHttpUtil.getIdentity(),
request.getClass().getName(),
JsonFormatUtil.printToString(request));
throw new PortalException(MSResultCode.MICROSERVICE_RETURN_NULL,
(" CompositeServiceEngine.getCompositeResponse(), Call microservice error, return null, " +
"requestClassName:{}")
.concat(request.getClass().getName()));
}

int code = apiResult.getCode();

if (LOGGER.isDebugEnabled())
{
LOGGER.debug(
"Exit CompositeServiceEngine.getCompositeResponse(), identityId:{}, requestClassName:{}, result:{}",
CommonHttpUtil.getIdentity(),
request.getClass().getName(),
code);
}
return String.valueOf(code);
}

}

　　这里有3个默认方法，一个抽象方法，抽象方法返回对象ApiResult<M>。我们来看看如果用匿名内部类怎么写：

CompositeServiceMethodInvoker<GetBookFeeDescResponse, GetBookFeeDescRequest> getBooFeeDescMethodInvoker =
new CompositeServiceMethodInvoker<GetBookFeeDescResponse, GetBookFeeDescRequest>(){

public ApiResult<GetBookFeeDescResponse> invoke(InvokeContext context, GetBookFeeDescRequest request)
{
ServiceController controller = createRpcController("getBookFeeDesc", context);
ApiResult<GetBookFeeDescResponse> result = new ApiResult<GetBookFeeDescResponse>(controller);
stub.getBookFeeDesc(controller, request, result);
return result;
}};

　　注意这里的泛型已经用具体类型替换了。如果我们使用lambda表达式，那么可以这么写：

CompositeServiceMethodInvoker<GetBookFeeDescResponse, GetBookFeeDescRequest> getBooFeeDescMethodInvoker =
(InvokeContext context, GetBookFeeDescRequest request) -> {
ServiceController controller = createRpcController("getBookFeeDesc", context);
ApiResult<GetBookFeeDescResponse> result = new ApiResult<GetBookFeeDescResponse>(controller);
stub.getBookFeeDesc(controller, request, result);
return result;
};

　　现在再来看这样一种情况，如果我们刚好在某个类中已经实现了lambda所指代的代码块，比如有这么一个类BookProductConsumer：

public class BookProductConsumer
extends  ServiceConsumer
{

public ApiResult<GetBookFeeDescResponse> getBookFeeDesc(InvokeContext context,
GetBookFeeDescRequest request) {
ServiceController controller = createRpcController("getBookFeeDesc",context);
ApiResult<GetBookFeeDescResponse> result = new ApiResult<GetBookFeeDescResponse>(controller);
stub.getBookFeeDesc(controller, request, result);
return result;
}
}

　　这里的getBookFeeDesc方法返回了ApiResult对象（这里接口里的泛型M已经具体为GetBookFeeDescResponse对象了）。我们可以看到，变量getBooFeeDescMethodInvoker所指代的方法块已经定义在了BookProductConsumer类的getBookFeeDesc方法中，所以使用方法引用来替换原来的lambda表达式：

CompositeServiceMethodInvoker<GetBookFeeDescResponse, GetBookFeeDescRequest> getBooFeeDescMethodInvoker = BookProductConsumer::getBookFeeDesc;

　　这就是类的方法引用，根据方法调用的不同情况，还有对象的方法引用、类的静态方法引用、类的构造方法引用。