ZK Client side trigger click on button

package j1oi61tg$v2;

import java.util.*;
import org.zkoss.bind.annotation.Command;
import org.zkoss.bind.annotation.NotifyChange;
import org.zkoss.zk.ui.util.Clients;
import org.zkoss.zul.ListModel;
import org.zkoss.zul.ListModelList;
public class TestVM {
	public String getMsg () {
		return "open upload dialog by client side programming";
	}
	public String getMsgTwo () {
		return "open upload dialog by javascript that executed by server";
	}
	public String getOverrideClick () {
		return "function (evt) {this.$doClick_(evt);jq('$btn').next().find('input').click();}";
	}
	@Command
	public void openUploadDialog () {
		System.out.println(" open upload");
		Clients.evalJavaScript("jq('$btn').next().find('input').click();");
	}
 
}

<zk xmlns:w="client">
	<window apply="org.zkoss.bind.BindComposer" viewModel="@id('vm') @init('j1oi61tg$v2.TestVM')">
	    <label value="@load(vm.msg)">
	    	<attribute w:name="doClick_">
	    		function (evt) {
	    			// call the original doClick_ function
	    			this.$doClick_(evt);
	    			// click the upload button by JQuery
	    			jq('$btn').next().find('input').click();
	    		}
	    	</attribute>
	    </label>
	    <div></div>
	    <label value="@load(vm.msgTwo)" onClick="@command('openUploadDialog')"></label>
	    <div></div>
	    <button id="btn" label="Upload" upload="true" />
	</window>
</zk>

package j1oi61tg$v3;

import java.util.*;
import org.zkoss.bind.annotation.Command;
import org.zkoss.bind.annotation.NotifyChange;
import org.zkoss.zk.ui.util.Clients;
import org.zkoss.zul.ListModel;
import org.zkoss.zul.ListModelList;
public class TestVM {
	public String getMsg () {
		return "open upload dialog by client side programming";
	}
	public String getMsgTwo () {
		return "open upload dialog by javascript that executed by server";
	}
	public String getOverrideClick () {
		return "function (evt) {this.$doClick_(evt);jq('$btn').next().find('input').click();}";
	}
	@Command
	public void openUploadDialog () {
		System.out.println(" open upload");
		Clients.evalJavaScript("jq('$btn').next().find('input').click();");
	}
 
}

<zk xmlns:w="client">
	<style>
		.z-button *,
		.z-upload * {
			background: transparent !important;
			margin: 0 !important;
			border: 0 !important;
			cursor: default !important;
		}
		.z-button-cm {
			display: none;
		}
	</style>
	<window apply="org.zkoss.bind.BindComposer" viewModel="@id('vm') @init('j1oi61tg$v3.TestVM')">
		<label value="@load(vm.msg)">
			<attribute w:name="doClick_">
				function (evt) {
					// call the original doClick_ function
					this.$doClick_(evt);
					//---- update domTarget and domEvent as need ----//
					// evt.domEvent = THE_NEW_EVENT;                 //
					// evt.domTarget = THE_NEW_TARGET;               //
					//---- ------------------------------------- ----//
					// click the upload button by JQuery
					jq('$btn').next().find('input').click();
				}
			</attribute>
		</label>
		<div></div>
		<label value="@load(vm.msgTwo)" onClick="@command('openUploadDialog')"></label>
		<div></div>
		<div style="position: relative; cursor: default;" height="30px"
			onMouseOver="uploadDiv.setParent(self);">
			<label value="open upload by real click on it" />
		</div>
		<div style="position: relative; cursor: default;" height="30px"
			onMouseOver="uploadDiv.setParent(self);">
			<label value="also open upload by real click on it" />
		</div>
		<div style="position: relative; cursor: default;" height="30px"
			onMouseOver="uploadDiv.setParent(self);">
			<label value="still open upload by real click on it" />
		</div>
		<div id="uploadDiv" style="position: absolute; left: 0px; top: 0px; z-index: 100;">
			<button id="btn" label="Upload" upload="true"
				width="200px" height="15px">
				<attribute name="onUpload"><![CDATA[
					import org.zkoss.util.media.Media;
					Media[] media = event.getMedias();
					if (media != null) { // show the infprmation of updated media
						StringBuilder sb = new StringBuilder("");
						for (int i = 0; i < media.length; i++) {
							sb.append(media[i].getName() + ", "+media[i].getByteData().length +" bytes\n");
						}
						result.setValue(sb.toString());
					}
					]]></attribute>
			</button>
		</div>
		result: <label id="result" />
	</window>
</zk>