1、前端vue中axios调用后台服务的代码：

           onsubmit(){
                this.axios.post("/api/leave/leaveApply",{
                    data:{
                        type:this.leaveForm.type,
                        startTime:this.leaveForm.startTime,
                        endTime:this.leaveForm.endTime,
                        reason:this.leaveForm.reason,
                        userId:'user1',
                        pdId:'myProcess_1:2:15004',
                    }
                }).then((res)=>{
                    alert(res.data);
                }).catch((res)=>{
                    alert("发起请假失败");
                })
            }

2、http请求参数如下：

3、springboot后台controller代码如下：

@RequestMapping("/leaveApply")
    @ResponseBody
    public String leaveApply(@RequestBody String leaveForm){
        System.out.println("leaveForm:"+leaveForm);
        if(StringUtils.isEmpty(leaveForm)){
            return "请假表单数据不允许为空";
        }
        JSONObject jsonObject=JSON.parseObject(leaveForm);
        System.out.println("jsonObj:"+jsonObject);
        LeaveForm leaveFormObj=jsonObject.toJavaObject(LeaveForm.class);
        System.out.println("leaveFormObj:"+leaveFormObj);
        activittiService.startProcess(leaveFormObj.getPdId(),leaveFormObj.getUserId());
        return "leaveApply";
    }

4、访问controller，后台控制台打印如下信息：

leaveForm:{"data":{"type":"2","startTime":"","endTime":"","reason":"啊","userId":"user1","pdId":"myProcess_1:2:15004"}}
jsonObj:{"data":{"reason":"啊","startTime":"","endTime":"","pdId":"myProcess_1:2:15004","type":"2","userId":"user1"}}
leaveFormObj:LeaveForm{type=null, startTime=null, endTime=null, reason='null', userId='null', pdId='null'}
2018-11-26 14:23:53.545 ERROR 5204 --- [nio-8088-exec-3] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.activiti.engine.ActivitiIllegalArgumentException: processDefinitionKey and processDefinitionId are null] with root cause

org.activiti.engine.ActivitiIllegalArgumentException: processDefinitionKey and processDefinitionId are null
	at org.activiti.engine.impl.cmd.StartProcessInstanceCmd.execute(StartProcessInstanceCmd.java:89) ~[activiti-engine-5.21.0.jar:5.21.0]
	at org.activiti.engine.impl.cmd.StartProcessInstanceCmd.execute(StartProcessInstanceCmd.java:37) ~[activiti-engine-5.21.0.jar:5.21.0]
	at org.activiti.engine.impl.interceptor.CommandInvoker.execute(CommandInvoker.java:24) ~[activiti-engine-5.21.0.jar:5.21.0]

原因分析：
根据打印的信息可知，json字符串转jsonObject成功了，但jsonObject转javaBean失败了，重点查看jsonObject打印的信息：jsonObj:{"data":{"reason":"啊","startTime":"","endTime":"","pdId":"myProcess_1:2:15004","type":"2","userId":"user1"}}
正常的普通对象是用一对{}包裹的，而这里的格式明显不对，多了一层data{}包裹，由此可见是json字符串的格式有问题，
再返回前台查看传递的数据格式如下：

原来是这里用data包裹数据，在这里修改为下面这样：

再次访问controller，后台打印一下信息：

leaveForm:{"type":"1","startTime":"2018-11-26T06:39:44.560Z","endTime":"2018-11-26T06:39:48.389Z","reason":"啊啊","userId":"user1","pdId":"myProcess_1:2:15004"}
jsonObj:{"reason":"啊啊","startTime":"2018-11-26T06:39:44.560Z","endTime":"2018-11-26T06:39:48.389Z","pdId":"myProcess_1:2:15004","type":"1","userId":"user1"}
leaveFormObj:LeaveForm{type=1, startTime=Mon Nov 26 14:39:44 CST 2018, endTime=Mon Nov 26 14:39:48 CST 2018, reason='啊啊', userId='user1', pdId='myProcess_1:2:15004'}

可以看出，jsonObject（jsonObj）已经没有data{}了，而且也成功转为javaBean（leaveFormObj），问题解决。