Is there an eval() function in Java?

java

dynamic-code-execution

security-risks

expression-parsers

byAlex Kataev·Feb 25, 2025

The javax.script.ScriptEngineManager class in Java allows the evaluation of JavaScript expressions. Here's a brief example:

import javax.script.*;

public class QuickEval {
    public static void main(String[] args) throws ScriptException {
        // Who said Java and JavaScript are not friends?
        ScriptEngine engine = new ScriptEngineManager().getEngineByName("nashorn");
        System.out.println(engine.eval("10 * 2 + 1"));  // Outputs: 21, not 2010!
    }
}

Remember: This usage of JavaScript doesn't mean you can execute Java code directly. For that, you'd have to use the Java Compiler API or third-party libraries like Janino or BeanShell.

Comprehensive guide to Java's eval()

Here's an in-depth look at dynamic code execution, alternatives, and risky elements tied to eval()-like functions in Java.

Security risks of eval

Exploiting eval() risks your code to a wide range of injection attacks. Using the JavaScript ScriptEngine is relatively safe, but untrusted input must be handled with care. With the Java Compiler API, the risk is more pronounced due to the inherent code execution dangers.

Crafting secure expression parsers

Secure alternatives include building a secure parser, such as a recursive-descent parser or a finite-state machine, which are safer and simpler. For dealing with complex expressions, use the ANTLR tool, although it requires a learning curve.

Enlisting third-party libraries

If you need advanced features, explore third-party libraries like Exp4j (specifically for mathematical expressions) or JEL (Java Expression Language) for a holistic language evaluation.

Post-Nashorn era

With the deprecation of Nashorn, you would need to evaluate replacements for embedding JavaScript or opt for scripting API-compliant languages like Groovy for dynamic scripting.

JShell (Java 9 and above)

Java 9 introduced JShell, enabling the evaluation of Java expressions in a REPL (Read-Eval-Print Loop). However, please note, although JShell provides robust capabilities, it is more suited for interactive development over production.

Design patterns to the rescue

For flexibility sans eval(), apply Strategy, Command, or Builder design patterns. These alternatives run code that can change at runtime while maintaining strict control.

Developing a desktop calculator

Need a real-world example? Think desktop calculator. To evaluate expressions, you might implement the shunting-yard algorithm, which interprets infix notation.

Visualization

Java without a native eval() is equivalent to a toolbox missing one tool:

Java's Toolbox 🧰: [Hammer 🔨, Pliers 🛠, Wrench 🔧, Tape Measure 📏]
Missing Tool: Eval Screwdriver 🪛 (Not in the box!)

To enable eval() in Java, you'd craft the missing tool with:

DIY Screwdriver 🪛:
1. ScriptEngineManager 🧙‍♂️
2. JavaScript Engine 📜
3. Eval-like Execution 🎭

Remember: Treat dynamic execution as carefully as handling a self-crafted tool.

Compiling Java source to bytecode

To run dynamic Java code, you may compile Java source to bytecode at runtime. However, beware, you'd need a comprehensive understanding of classloaders and JVM security.

Utilizing handy expression evaluator libraries

Given the vast range of third-party libraries, you can choose from MVEL, JEval, or specialize in libraries listed in numerous linked answers on Stack Overflow.

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