how to call json api by java code

HI all:

i want to used joget workflow in my java application,and used myself form

so , i shoud invoke the completeActivity json api to complete the activity

the full requesturl like this:

http://localhost:8080/jw/web/json/workflow/assignment/completeWithVariable/activityId?var_externalId=****;

but when i call this url through java code :new Url() or httpclient

it's always occure the error:500 server error

but if i call the url by browser,example ie,and there will result return.

anybody can tell me what should i do if i want to call the url in java application ?