JavaCodeGenerator.java

import java.io.BufferedWriter;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.nio.charset.StandardCharsets;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.UUID;

public class JavaCodeGenerator {
    private static List<String> generateSource(String className, Map<String, String> fields)
    throws IllegalArgumentException {
        if (!validName(className)) { throw new IllegalArgumentException("className"); }
        if (fields == null) { throw new IllegalArgumentException("fields"); }
        final List<String> source = new ArrayList<>();
        source.add(String.format("public class %s {", className));
        for (Map.Entry<String, String> pair : fields.entrySet()) {
          if (!validName(pair.getKey())) { throw new IllegalArgumentException("fields"); }
          source.add("");
          source.add(String.format("    public %s %s;", pair.getValue(), pair.getKey()));
        }
        source.add("}");
        return source;
    }

    private static String buildClassFile(String className, List<String> source, String path)
    throws IllegalArgumentException, IOException, InterruptedException {
        if (!validName(className)) { throw new IllegalArgumentException("className"); }
        if (source == null || source.isEmpty()) { throw new IllegalArgumentException("source"); }
        if (path == null || path.isEmpty()) { throw new IllegalArgumentException("path"); }
        final File directory = new File(path,  UUID.randomUUID().toString());
        directory.mkdir();
        final File sourceFile = new File(directory, className + ".java");
        try (
            FileOutputStream fos = new FileOutputStream(sourceFile);
            OutputStreamWriter osw = new OutputStreamWriter(fos, StandardCharsets.UTF_8);
            BufferedWriter bw = new BufferedWriter(osw)
        ) {
            for (String line : source) {
                bw.write(line);
                bw.newLine();
            }
        } catch (IOException ex) {
            throw ex;
        }
        if (sourceFile.exists()) {
            final String fs = System.getProperty("file.separator");
            final Process process = new ProcessBuilder(
                System.getenv("JAVA_HOME") + fs + "bin" + fs + "javac",
                "-source",
                "8",
                "-target",
                "8",
                "-encoding",
                "utf8",
                "-cp",
                directory.getAbsolutePath(),
                "-d",
                directory.getAbsolutePath(),
                sourceFile.getAbsolutePath()
            ).start();
            final int exitCode = process.waitFor();
            sourceFile.delete();
            if (exitCode != 0) {
                final File classFile = new File(directory, className + ".class");
                if (classFile.exists()) {
                    classFile.delete();
                }
                directory.delete();
                return "";
            }
        }
        return directory.getAbsolutePath();
    }

    private static boolean validName(String name) {
        if (name == null || name.isEmpty()) return false;
        final char[] chars = name.toCharArray();
        if (chars[0] != '_' && !Character.isLetter(chars[0])) return false;
        for (int i = 1; i < chars.length; i++) {
            if (chars[i] != '_' && !Character.isLetter(chars[i]) && !Character.isDigit(chars[i])) return false;
        }
        return true;
    }

    private static void usingClass(String className, String path) {
        // User user = new User();
        // user.id = 1;
        // user.name = "admin";
        // user.password = "qwerty";
        // System.out.printf(
        //     "{ 'id': '%d', 'name': '%s', 'password': '%s'}",
        //     user.id,
        //     user.name,
        //     user.password
        // );
        // System.out.println();
    }

    public static void main(String[] args) {
        try {
            final Map<String, String> fields = new HashMap<>();
            fields.put("id", "Integer");
            fields.put("name", "String");
            fields.put("password", "String");
            final List<String> source = generateSource("User", fields);
            final String path = buildClassFile("User", source, System.getProperty("user.dir"));
            if (!path.isEmpty() && (new File(path, "User.class")).exists()) {
                System.out.println("Class User created in: " + path);
                usingClass("User", path);
                (new File(path, "User.class")).delete();
                (new File(path)).delete();
            }
        } catch (Exception ex) {
            System.err.println("[ERROR] " + ex.getMessage());
        }
    }
}