How in Java do I inject code during execution time?

Question

I'm working on a java spring app which I need to provide the ability to add new code without altering standard code.

My challenges:

1. Store "custom" code in a database
2. Inject code at execution time into existing standard code
3. Isolate custom code so that two users do not execute each others code.

In other words, what I am looking for is upon method execution, look to the db if user has custom code for this method, if no, execute standard code; if yes, inject code, execute modified method then dispose of custom code.

I've looked at AspectJ and Javassist. I have a process working with AspectJ but I can only get it to work if the custom code is in it's own jar and added at build/runtime. I haven't spent much time with Javassist but from the docs, it looks like i'm in the same boat.

Here's an extremely simplified example of what i'm looking to do.

@RestController
@RequestMapping("/test")
public class TestController {
   @RequestMapping(value = "/test", method = RequestMethod.GET)
   public String test() {

      String results = "";

      results = "foo";

      //<-- insert custom code here (if exists) each time method is invoked

      results += "bar";

      return results;
   }
}

Answer 1

As for Java scripting support according to JSR-223, here is some introductory documentation for you.

By default the Nashorn JavaScript engine is contained in the JRE, but since Java 11 you get a warning that it is deprecated and will be removed in the future. This is not a big problem because you will still be able to use it and there are also other JSR-223-compatible scripting languages available.

This example shows how to use

• JavaScript,
• Groovy and
• Lua

scripts from Java. Please make sure that you add the corresponding

• Groovy (e.g. groovy-all-2.4.14.jar) and
• Lua (e.g. luaj-jse-3.0.1.jar)

libraries to your classpath if you want to use the additional two languages and not just the built-in JS engine.

The scripts in each language demonstrate the same basics things, i.e. how to

• call a method on a Java object (File.getCanonicalPath()) from the script,
• how to convert a string to upper case,
• how to repeat a string (in JS we have to define our own prototype method for that because Nashorn does not implement the repeat(n) method by itself).

package de.scrum_master.app;

import java.io.File;
import javax.script.*;

public class Application {
  private static final ScriptEngineManager MANAGER = new ScriptEngineManager();

  public static void main(String[] args) throws ScriptException {
    listScriptingEngines();
    System.out.println(runScript("JavaScript", "print(file.getCanonicalPath()); String.prototype.repeat = function(num) { return new Array(num + 1).join(this) }; (text.toUpperCase() + '_').repeat(2)"));
    System.out.println(runScript("Groovy", "println file.canonicalPath; (text.toUpperCase() + '_') * 2"));
    System.out.println(runScript("lua", "print(file:getCanonicalPath()); return string.rep(string.upper(text) .. '_', 2)"));
  }

  public static void listScriptingEngines() {
    for (ScriptEngineFactory factory : MANAGER.getEngineFactories()) {
      System.out.printf("Script Engine: %s (%s)%n", factory.getEngineName(), factory.getEngineVersion());
      System.out.printf("  Language: %s (%s)%n", factory.getLanguageName(), factory.getLanguageVersion());
      factory.getNames().stream().forEach(name -> System.out.printf("    Alias: %s%n", name));
    }
  }

  public static String runScript(String language, String script) throws ScriptException {
    ScriptEngine engine = MANAGER.getEngineByName(language);
    // Expose Java objects as global variables to script engine
    engine.put("file", new File("/my/" + language + "/test.txt"));
    engine.put("text", language);
    // Use script result for generating return value
    return engine.eval(script) + "suffix";
  }
}

If you run the program with groovy-all on the classpath, the output should look like this:

Script Engine: Groovy Scripting Engine (2.0)
  Language: Groovy (2.4.13)
    Alias: groovy
    Alias: Groovy
Script Engine: Luaj (Luaj-jse 3.0.1)
  Language: lua (5.2)
    Alias: lua
    Alias: luaj
Script Engine: Oracle Nashorn (1.8.0_211)
  Language: ECMAScript (ECMA - 262 Edition 5.1)
    Alias: nashorn
    Alias: Nashorn
    Alias: js
    Alias: JS
    Alias: JavaScript
    Alias: javascript
    Alias: ECMAScript
    Alias: ecmascript
C:\my\JavaScript\test.txt
JAVASCRIPT_JAVASCRIPT_suffix
C:\my\Groovy\test.txt
GROOVY_GROOVY_suffix
C:\my\lua\test.txt
LUA_LUA_suffix

Of course in your own code you should replace the inline JS, Groovy or Lua code with something user-specific loaded from your code snippet database. You should also think about which variables you want to expose to the scripting engine and document it for your users.