Execute Java code that is stored in the database

Question

I have Java code that is regularly pushed to the database (it is too complicated to explain why it is in the database, and that would just turn the focus away from the main

Answer 1

Any Java program is correct Groovy program. So you can add Groovy dependency to your project and then using GroovyShell execute your code:

GroovyShell shell = new GroovyShell();
shell.evaluate(code);

So in your case:

GroovyShell shell = new GroovyShell();
shell.evaluate("int i = 10;\n" +
        "int j = 2;\n" +
        "int k = i*j;\n" +
        "System.out.println(\"Result is \" + k);");

And output:

Result is 20

Or you can use ScriptEngineManager (more common way):

ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("groovy");
engine.eval("int i = 10;\n" +
        "int j = 2;\n" +
        "int k = i*j;\n" +
         "System.out.println(\"Result is \" + k);");

But anyway, you need to add Groovy to your depedencies.

Answer 2

You could build the java source code and then load the java class using reflection. Therefore, add a new Servlet to your HTTP Server, call it getJar, and put the following in the doGet method (assuming you have already created the DB connection):

File tmp = File.createTempFile("java-code-", ".java");
PrintWriter out = new PrintWriter(new FileWriter(tmp), true);
String classname = tmp.getName().substring(0, tmp.getName().length()-5);
out.println("public class " + classname);
out.println("{");
out.println("    public static void executeCode ()");
out.println("    {");
out.println(connection.executeQuery("SELECT java-code FROM yourDb WHERE name='"
        + request.getParameter("name").replace("'", "''") + "';");
out.println("    }");
out.println("}");
out.close();
Runtime.getRuntime().exec("javac \"" + tmp.getAbsolutePath() + "\"").waitFor();
Runtime.getRuntime().exec("jar cfe \"" + tmp.getAbsolutePath() + ".jar\" \""
         + classname + "\" \"" + tmp.getAbsolutePath() + "\"");
InputStream in = new InputStream(new FileInputStream(tmp.getAbsolutePath() + ".jar"));
int read; byte[] buf = new byte[4096];
while ((read = in.read(buf)) != null)
    response.getOutputStream().write(buf, 0, read);
in.close();

Now, you can receive it and create a ClassLoader for it, and then execute it:

ClassLoader cl = new URLClassLoader("http://localhost/getJar?name=whatever");
Class c = cl.loadClass(classname);
c.getMethod("executeCode").invoke(null);