Lambda expressions in JSP files will not compile (continued)

Question

I found here the following article: Lambda expressions in JSP files will not compile

So I added compilerSourceVM and compilerTargetVM to conf/web.xml in Tomcat. (not in the project web.xml).

So in general I can use stream, but for (some?) lambda expression the JSP will not be compiled by tomcat.

The following are examples. I want get the lambda working, I'm not looking for better coding in first example.

Simple example (working):

    <p>
    Cookies:<br />
    <%
        Cookie[] cookies = request.getCookies();
        if ( cookies == null || cookies.length == 0 ) {
            out.println("No cookies found<br/>");
        } else {
            out.println("Cookies found size=" + cookies.length + "<br/>");
            List<Cookie> cList = Arrays.stream(request.getCookies()).collect(Collectors.toList());
            for ( Cookie c : cList ) {
                out.println("C: " + c + "<br/>");
            }
            out.println("Cookies found size=" + cookies.length + "<br/>");
        }
    %>
</p>

Simple example (not working):

Instead to have a for loop and collecting in a list, I will use the Stream::forEach with the lambda expression like:

  Arrays.stream(request.getCookies()).forEach(cookie -> {out.println("Cookie information here<br/>");});

In Eclipse the JSP will be compiled and don't show some errors.

Deploying to Tomcat and calling the URL, I'm getting:

    org.apache.jasper.JasperException: Unable to compile class for JSP: 
An error occurred at line: [53] in the jsp file: [/report.jsp]
Unhandled exception type IOException
50:             } else {
51:                 out.println("Cookies found size=" + cookies.length + "<br/>");
52:                 Arrays.stream(request.getCookies()).forEach(cookie -> {
53:                     out.println("Cookie information here<br/>");
54:                 });  

Any hints?

Thanks in advance.

Edit:

I switched to Tomcat 9. Same problem. I found also some hints, that Java 1.8 + stream + lambdas should work in Tomcat8+9.

So I changed the lambda expression like:

Arrays.stream(request.getCookies()).forEach(cookie -> {System.out.println("Cookie information here<br/>");});

So I replaced "out.println" to "System.out.println". Now the stream and lambda are working. Println going to console as expected. Also added some ".filter". Also working fine.

Now the question. Why I can't use out.println or out.write in the lambda expression. In the same JSP the out.println is working in the for-each loop showing in the example above. Any idea?

Edit:

Found the next problem, this is pure Java problem, so when you edit a JSP in Eclipse. Eclipse is not showing this problem in the editor.

a.) out.write/out.println will throw a checked exception, so I need a try/catch, because lambda's not allow a checked exception.

b.) out Variable must be final. So you need a final JspWriter myOut = out;

c.) You may have to import the JSP API lib into our project. Gradle example: providedCompile 'javax.servlet.jsp:javax.servlet.jsp-api:2.3.0'

Answer 1

Can you try adding the below init parameters in your web.xml,

    <init-param>
        <param-name>compilerSourceVM</param-name>
        <param-value>1.8</param-value>
    </init-param>
    <init-param>
        <param-name>compilerTargetVM</param-name>
        <param-value>1.8</param-value>
    </init-param>