Spring in Practice

Willie Wheeler's Spring blog

How to Run JavaScript From Java

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Java 6 comes with the Rhino JavaScript engine, which makes it easy to run JavaScript from inside your Java app. There are different situations in which you might want to do this. Chapter 9 of Spring in Practice affords a good example. There we’re implementing a rich-text comment engine based on the WMD editor that Stack Overflow uses. We have a showdown.js script that maps Markdown to HTML, and we want to run it in two places:

  • on the client to present a preview pane
  • on the server to store the Markdown as HTML for easy presentation (as opposed to running the script against every comment dynamically)

Here’s how we can run it on the server:

import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;


public final class RichTextFilter implements TextFilter {
    @Inject private File showdownJsFile;

    private String markdownToHtml(String markdown) {
        try {
            ScriptEngineManager mgr = new ScriptEngineManager();
            ScriptEngine engine = mgr.getEngineByName("JavaScript");
            engine.eval("var markdown = '" + markdown + "';");
            engine.eval("var converter = new Showdown.converter();");
            engine.eval("var html = converter.makeHtml(markdown);");
            return (String) engine.get("html");
        } catch (ScriptException e) {
            // Shouldn't happen unless somebody breaks the script
            throw new RuntimeException(e);

    ... other stuff ...

To see how to inject the File into the class in Spring, see my post Injecting a file from the classpath into a bean.