毎回URLConnectionを開き直してるのでセッション管理で発行されるCookieが維持できない
なのでCookieManagerを使ってCookieを受け入れるようにする
json_client.groovy
import groovy.swing.SwingBuilder
import groovy.json.JsonOutput
import groovy.beans.Bindable
import javax.swing.*
import java.awt.*
import java.net.*

def manager = new CookieManager()
manager.cookiePolicy = CookiePolicy.ACCEPT_ALL
CookieHandler.default = manager

@Bindable
class Model {
	String method = 'GET'
	String url = ''
	String params = ''
	String json = ''
}
def model = new Model()

def tabPane

def prettyPrint = { text ->
	def result = JsonOutput.prettyPrint(text)
	(result =~ /\\u[0-9a-zA-Z]{4}/).each { match ->
		def code = match[2..5]
		def encTxt = new String([Integer.parseInt(code, 16)] as int[], 0, 1)
		result = result.replaceAll(/\\u${code}/, encTxt)
	}
	result
}

def sendRequest = {
	model.json = ''
	
	def method = model.method
	def url = model.url
	def params = URLEncoder.encode(model.params, 'utf-8').replaceAll('%3D', '=').replaceAll('%26', '&')
	if (method == 'GET') {
		url = "${url}?${params}"
	}
	
	def conn = url.toURL().openConnection()
	conn.requestMethod = method
	if (method == 'POST') {
		conn.doOutput = true
		conn.outputStream << params
	}
	
	model.json = prettyPrint(conn.inputStream.getText('utf-8'))
	tabPane.selectedIndex = 1
}

new SwingBuilder().frame(title:'jsonを取得、表示するアプリケーション', defaultCloseOperation:JFrame.EXIT_ON_CLOSE,
						size: [800, 600], show:true) {
	lookAndFeel('nimbus')
	borderLayout()
	hbox(constraints: BorderLayout.NORTH) {
		comboBox(items:['GET', 'POST'], selectedItem:bind(source: model, sourceProperty: 'method', mutual: true))
		textField(text:bind(source: model, sourceProperty: 'url', mutual: true))
		button(text:'送信', actionPerformed:sendRequest)
	}
	tabPane = tabbedPane(constraints: BorderLayout.CENTER) {
		scrollPane(name:'パラメータ') {
			textArea(text:bind(source: model, sourceProperty: 'params', mutual: true))
		}
		scrollPane(name:'レスポンス') {
			textArea(editable:false, text:bind {model.json})
		}
	}
}