確認しないで放置してたところを修正しつつ入出力をモデルにバインドする
bind()にmutual: trueを指定すると入力値がモデルに反映される
（双方向にバインドされる感じ）

json_client.groovy

import groovy.swing.SwingBuilder
import groovy.json.JsonOutput
import groovy.beans.Bindable
import javax.swing.*
import java.awt.*
import java.net.*

@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})
		}
	}
}