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Java – Lambda from String

Sometimes you may want to your application configuration file to define a function instead of a single value. In that case you should probably use Nashorn JavaScript engine that comes with Java 8, as advised by this stackoverflow answer. However, if for some reason you would like to stick to Java then this library might be a good choice for you.

Reasons to use it

LambdaFromString is a library that can generate a Java 8 lambda object at runtime from its code stored in a String.

  • Runtime behavior change – you can change the way your applicaton behaves at runtime, without recompiling it or even shutting it down.
  • Quick prototyping – you can quickly prototype different behaviors by just changing the code of the lambda stored in some file and making your application reload it on demand.
  • Wider configuration possibilities – you can let your users freely specify relation between some X and Y in the configuration file without constraining them by some predefined choices (let them write a code of Function<Integer,Integer> for example).
  • Simplicity – you can get what you would normally achieve by manually compiling a new class and loading it with a custom classloader in your application (but the library does it for you).

The library assumes that the lambda code is not malicious and doesn't validate it. Creating and executing lambdas based on external code can be really harmful so please make sure that it will be secure in your case.

Code examples

LambdaFactory lambdaFactory = LambdaFactory.get();

Function<Integer, Integer> increase = lambdaFactory.createLambda(
        "i -> i+1", new TypeReference<Function<Integer, Integer>>(){});
assertTrue(1 == increase.apply(0));

IntBinaryOperator multiply = lambdaFactory.createLambda(
        "(a,b) -> a*b", new TypeReference<IntBinaryOperator>(){});
assertEquals(1*2*3*4, IntStream.range(1,5).reduce(multiply).getAsInt());

Function<Integer, String> decorate = lambdaFactory.createLambda(
        "i -> \"ABC\"+i+\"DEF\"", new TypeReference<Function<Integer, String>>(){});
assertEquals("ABC101DEF", decorate.apply(101));

(You can visit tests directory to see additional examples.)

By default only java.util.function.* is imported by the class, as it is needed by the library itself. If you would like to import additional classes, you can specify imports in the configuration, as shown below. Please note that classes used in a lambda need to be available on the classpath used by the library, which is configurable. By default, the library uses the java.class.path system property.

Imports can be passed as Class<?> instances or strings (string form is the only way to use wildcards). Static imports are also supported and can be passed as strings.

LambdaFactory factory = LambdaFactory.get(
        LambdaFactoryConfiguration.get().withImports(BigDecimal.class));
BiFunction<BigDecimal, BigDecimal, BigDecimal> lambda = factory.createLambda(
        "(a,b) -> a.add(b)", new TypeReference<BiFunction<BigDecimal, BigDecimal, BigDecimal>>() {});
assertEquals(new BigDecimal("11"), lambda.apply(BigDecimal.ONE, BigDecimal.TEN));

createLambda throws a checked LambdaCreationException. If the exception is caused by compilation errors, it will contain a CompilationDetails instance with all messages from the compiler. Class source and compilation errors are also provided as the exception message, so they will appear in the stacktrace. It is the only exception that can be thrown by that method, all other runtime exceptions are caught and wrapped by this one. If you don't like checked exceptions you can call createLambdaUnchecked which is a proxy to createLambda that throws LambdaCreationRuntimeException instead.

Unfortunately the TypeReference class has to be subclassed when creating an instance. It looks like it's the only way to get generic type information at runtime and it's called super type tokens. The expected type of the lambda is used by the compiler for type inference. Thanks to that the string with lambda code doesn't have to specify types.

The compilation process takes time (on my laptop: first call ~1s, subsequent calls ~0.1s) so it probably should not be used in places where performance matters. The library is rather intended to be used once during the configuration reading process when the application starts. LambdaFactory instances are threadsafe.

Requirements and downloads

The library works with Java 8+. You can get it from Maven Central:

<dependency>
	<groupId>pl.joegreen</groupId>
	<artifactId>lambda-from-string</artifactId>
	<version>1.7</version>
</dependency>

It has two external Maven dependencies:

  • Apache Commons Lang. That dependency was added because java.lang.reflect.Type.toString() doesn't describe type parameters in a way that can be directly used for compilation and TypeUtils.toString(Type) has to be used instead.
  • Eclipse JDT Core Batch Compiler. Marked as 'optional' which means it should not be transitively downloaded by Maven. ECJ dependency was added because Java compiler is a part of JDK (located in tools.jar) and it's not available in pure JRE. When client applications were running on JRE then no Java compiler was available at runtime and the LambdaFromString library failed to compile lambda code. Eclipse ECJ makes it possible to use LambdaFromString even in cases when only JRE is available at runtime. Unfortunately ECJ works with this library only for Java 8 and not for higher versions.

Running with JDK

If you run your application on the JDK (instead of the JRE) the library should work out of the box.

Running with JRE and Eclipse ECJ

If you run your application on the JRE (without JDK) you can use the Eclipse ECJ compiler instead. The library will automatically try to use the Eclipse compiler if it doesn't find the JDK compiler. The ECJ dependency is marked as optional though, so you have to add it explicitly to your project:

<dependency>
	<groupId>org.eclipse.jdt.core.compiler</groupId>
	<artifactId>ecj</artifactId>
	<version>4.6.1</version>
</dependency>

How it works?

It actually compiles a new class using the Java Compiler API and some tricks to perform the whole compilation process in memory. The source that is compiled looks like this:

{IMPORTS}
public class LambdaFromStringHelper {
    public static {TYPE} getLambda() {return ({LAMBDA_CODE});}
}

The class is loaded by a custom class loader and then reflection is used to call 'getLambda' to get the actual lambda.

Class loader issues

In projects with a more complex class loader structure (for example Spring Boot based apps) the parent class loader that this library uses by default may be wrong. If this happens, JVM believes that classes used by the lambda code are not the same as classes in the project, even if the names and packages are identical. This can cause exceptions like this:

java.lang.ClassCastException: class com.test.MyClass cannot be cast to class com.test.MyClass
(com.test.MyClass is in unnamed module of loader org.springframework.boot.devtools.restart.classloader.RestartClassLoader @4872bd95; 
com.test.MyClass is in unnamed module of loader 'app')

Parent class loader can be configured manually to fix it:

LambdaFactoryConfiguration.get().withParentClassLoader(MyClass.class.getClassLoader());

Contribution

Issues and pull requests are welcome. By contributing, you agree to allow the project owner to license your work under the the terms of the MIT license.

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