I wrote a Java program whose filename was (intentionally) different from the class I wrote inside the file. The javac command failed as expected on both CMD and WSL. The java command however worked and ran my print statement. I wrote the code intentionally this way so there is no way it was a previously compiled version of the code. The following code was written in a file called "explainJava.java" (notice the filename is different from the class name).
public class explain{
public static void main(String[] args) {
System.out.println("Java is weird");
}
}
I've had to google this myself, but I think I've found an explanation in this article.
According to that source as of Java 11 java is capable of compiling a single source file into memory.
What I conclude from that: When the file is compiled into memory and not written to disk it obviously cannot have a file name. If there is no filename there is no such thing as a wrong filename, therefore the code executes.
Please also note that the restriction of having to name a file like the public class within that file is more of a design decision to make work for the compiler easier/ faster. It is not a physical restriction so to speak. Have a look at the following thread for more details.
If you put this code:
public class explain {
public static void main(String[] args) {
System.out.println("Java is weird");
}
}
into a file named explainJava.java, and then compile it with this:
javac explainJava.java
you will get an error that correctly informs you that your filename ("explainJava") and the class defined inside that file ("explain") do not match:
explainJava.java:1: error: class explain is public, should be declared in a file named explain.java
public class explain{
^
1 error
If you run this command:
$ java explainJava.java
Java is weird
you see expected output, because you're skipping the explicit compilation step (that is, you aren't running javac first) and instead relying on behavior introduced in Java 11 that allows you to compile+run in a single step. Here's an explanation: Does the 'java' command compile Java programs?
So the answer is to either:
rename your file to match the class, so change the filename to "explain.java", or
rename the class to match the file, change public class explain to be public class explainJava
I am trying to call a java script function from java code.
Here is my Java code
public static void main(String[] args) throws FileNotFoundException {
try {
/**
* To call a anonymous function from java script file
*/
ScriptEngine engine = new ScriptEngineManager()
.getEngineByName("javascript");
FileReader fr = new FileReader("src/js/MySpec.js");
engine.eval(fr);
} catch (ScriptException scrEx) {
scrEx.printStackTrace();
}
}
Here is my java script file:
(function() {
alert("Hello World !!!");
})();
But when I run main method of driver class it is giving me error as below:
Exception in thread "main" javax.script.ScriptException: sun.org.mozilla.javascript.internal.EcmaError: ReferenceError: "alert" is not defined. (<Unknown source>#2) in <Unknown source> at line number 2
at com.sun.script.javascript.RhinoScriptEngine.eval(RhinoScriptEngine.java:110)
at javax.script.AbstractScriptEngine.eval(AbstractScriptEngine.java:232)
at Java6RhinoRunner.load(Java6RhinoRunner.java:42)
at Java6RhinoRunner.main(Java6RhinoRunner.java:12)
What I know is that it need some script engine to execute it.
For that I added rhino.jar file in to my class path.But this is not working.
I an not getting how to solve this error.
Please help.Thanks in advance.
alert is not part of JavaScript, it's part of the window object provided by web browsers. So it doesn't exist in the context you're trying to use it in. (This is also true of setInterval, setTimeout, and other timer-related stuff, FYI.)
If you just want to do simple console output, Rhino provides a print function to your script, so you could replace alert with print. Your script also has access to all of the Java classes and such, so for instance java.lang.System.out.println('Hello'); would work from your JavaScript script (although it's a bit redundant with the provided print function). You can also make Java variables available to your script easily via ScriptEngine.put, e.g:
engine.put("out", System.out);
...and then in your script:
out.println('Hello from JavaScript');
...so that's a third way to do output from the script. :-)
See the discussion in the javax.script package documentation, in particular ScriptEngine#put, or for more complex cases, Bindings (and SimpleBindings) and ScriptContext.
I am getting into Clojure and Grammatical Evolution at the same time and discovered GEVA, which is a GE tool written in Java. I have no background in Java. So I don't have to re-invent the wheel, how can I integrate GEVA into Clojure?
I can execute the default script from the CLI with:
java -jar GEVA.jar -main_class Main.Run
The GEVA source directory has the following subdirectories with packages named after the directory:
Algorithm com Exceptions FitnessEvaluation Fractal Individuals Main Mapper Operator Parameter UI Util
The Main subdirectory has the following files and tutorial subdir:
AbstractRun.java Experiment.java Run.java State.java Tutorials
The following classes are in the Run.java file:
public class Run extends AbstractRun
public Run()
public void experiment(String[] args)
public void setup(String[] args)
private void setSeed()
public static void main(String[] args)
From what I can tell, the static method 'main' in class Main.Run is what I want to execute. However when I try to follow the Java interop instructions, I am getting errors.
clgeva.test=> (.Main.Run/main)
java.lang.Exception: No such namespace: .Main.Run (NO_SOURCE_FILE:5)
clgeva.test=> (.Main.Run/main "")
java.lang.Exception: No such namespace: .Main.Run (NO_SOURCE_FILE:6)
I'm obviously not importing the namespace correctly to run the main function.
user=> (import Main)
java.lang.ClassNotFoundException: Main (NO_SOURCE_FILE:1)
user=> (import Main.Run)
Main.Run
user=> (main)
java.lang.Exception: Unable to resolve symbol: main in this context (NO_SOURCE_FILE:3)
user=> (.Main.Run/main)
java.lang.Exception: No such namespace: .Main.Run (NO_SOURCE_FILE:4)
What the heck am I doing wrong? I am sure I'm just missing something obvious, but I've been banging my head on it for a little while and need some direction. Thanks.
First, make sure you are loading the GEVA lib correctly - see http://clojure.org/libs
I'm guessing, from the java.lang.ClassNotFoundException error, that GEVA is not on your classpath.
Then you want to look at the Clojure documentation for Java interop: http://clojure.org/java_interop
The syntax for calling a static method with no arguments (I'll use an example that everyone has access to) is:
=> (java.lang.System/currentTimeMillis)
1284574337322
The above form is preferred, but the following is also possible, and can be useful in macros:
=> (. java.lang.System currentTimeMillis)
1284574477369
In this case, "java.lang." may be omitted, but I'm including it to match your case more closely.
=> (System/currentTimeMillis)
1284574617771
Adding a dot to the first form will not work, whether with or without a space:
user=> (.System/currentTimeMillis)
java.lang.Exception: No such namespace: .System (NO_SOURCE_FILE:44)
user=> (. System/currentTimeMillis)
java.lang.IllegalArgumentException: Malformed member expression, expecting (. target member ...) (NO_SOURCE_FILE:45)
You have the right idea, but you do have some syntax errors.
clgeva.test=> (.Main.Run/main)
Should have a space between the dot operator and the Main.Run/main reference. As it is, it's trying to find something called ".Main.Run" which obviously doesn't exist. Do it like so:
clgeva.test=> (. Main.Run/main)
Assuming you have your classpath set up correctly, it should work.
Importing the class using (import ...) and calling (main) won't work because main is a static method.
This is my first post on the forum, hope all of you guys are well.
I've got a issue using JiST/SWANS, the ad hoc simulator in java within eclipse.
I managed to load the API, (as an external JAR ofcourse) but Im basically having a problem integrating the runtime of JiST within eclipse.
After running the hello world im usually getting a stackoverflowerror exception, since it may need modifications within the runtime.
import jist.runtime.JistAPI;
public class hello implements JistAPI.Entity {
/**
* #param args
*/
public static void main(String[] args) {
System.out.println("simulation start");
hello t = new hello();
t.myEvent();
}
public void myEvent()
{
JistAPI.sleep(1);
myEvent();
System.out.println("hello world, t=" + JistAPI.getTime());
}
}
the website is: http://jist.ece.cornell.edu/index.html
Thank you in advance!
Actually you need to run Main.java within jist.runtime. But before rigth click Main.java, properties, Run/Debug settings, New,Arguments and type your class name (plain name no .java needed) in Progam arguments. This will tell the jist interface to translate your code using the rewriter and run it.
Examples:
To run hello.java type "hello"
To run aodvsim.java type: "jist.swans.Main driver.aodvsim"
If there are arguments needed type them after the clas name like: "jist.swans.Main driver.aodvsim -n 25 -f 2000x2000 -a grid:5x5 -t 10,600,60"
Wilmer Arellano
How well does SWANS work? Given that the documentation and code date back to 2005, I am not sure if this is the best platform to use.
Most of the top google hits for "calling clojure from java" are outdated and recommend using clojure.lang.RT to compile the source code. Could you help with a clear explanation of how to call Clojure from Java assuming you have already built a jar from the Clojure project and included it in the classpath?
Update: Since this answer was posted, some of the tools available have changed. After the original answer, there is an update including information on how to build the example with current tools.
It isn't quite as simple as compiling to a jar and calling the internal methods. There do seem to be a few tricks to make it all work though. Here's an example of a simple Clojure file that can be compiled to a jar:
(ns com.domain.tiny
(:gen-class
:name com.domain.tiny
:methods [#^{:static true} [binomial [int int] double]]))
(defn binomial
"Calculate the binomial coefficient."
[n k]
(let [a (inc n)]
(loop [b 1
c 1]
(if (> b k)
c
(recur (inc b) (* (/ (- a b) b) c))))))
(defn -binomial
"A Java-callable wrapper around the 'binomial' function."
[n k]
(binomial n k))
(defn -main []
(println (str "(binomial 5 3): " (binomial 5 3)))
(println (str "(binomial 10042 111): " (binomial 10042 111)))
)
If you run it, you should see something like:
(binomial 5 3): 10
(binomial 10042 111): 49068389575068144946633777...
And here's a Java program that calls the -binomial function in the tiny.jar.
import com.domain.tiny;
public class Main {
public static void main(String[] args) {
System.out.println("(binomial 5 3): " + tiny.binomial(5, 3));
System.out.println("(binomial 10042, 111): " + tiny.binomial(10042, 111));
}
}
It's output is:
(binomial 5 3): 10.0
(binomial 10042, 111): 4.9068389575068143E263
The first piece of magic is using the :methods keyword in the gen-class statement. That seems to be required to let you access the Clojure function something like static methods in Java.
The second thing is to create a wrapper function that can be called by Java. Notice that the second version of -binomial has a dash in front of it.
And of course the Clojure jar itself must be on the class path. This example used the Clojure-1.1.0 jar.
Update: This answer has been re-tested using the following tools:
Clojure 1.5.1
Leiningen 2.1.3
JDK 1.7.0 Update 25
The Clojure Part
First create a project and associated directory structure using Leiningen:
C:\projects>lein new com.domain.tiny
Now, change to the project directory.
C:\projects>cd com.domain.tiny
In the project directory, open the project.clj file and edit it such that the contents are as shown below.
(defproject com.domain.tiny "0.1.0-SNAPSHOT"
:description "An example of stand alone Clojure-Java interop"
:url "http://clarkonium.net/2013/06/java-clojure-interop-an-update/"
:license {:name "Eclipse Public License"
:url "http://www.eclipse.org/legal/epl-v10.html"}
:dependencies [[org.clojure/clojure "1.5.1"]]
:aot :all
:main com.domain.tiny)
Now, make sure all of the dependencies (Clojure) are available.
C:\projects\com.domain.tiny>lein deps
You may see a message about downloading the Clojure jar at this point.
Now edit the Clojure file C:\projects\com.domain.tiny\src\com\domain\tiny.clj such that it contains the Clojure program shown in the original answer. (This file was created when Leiningen created the project.)
Much of the magic here is in the namespace declaration. The :gen-class tells the system to create a class named com.domain.tiny with a single static method called binomial, a function taking two integer arguments and returning a double. There are two similarly named functions binomial, a traditional Clojure function, and -binomial and wrapper accessible from Java. Note the hyphen in the function name -binomial. The default prefix is a hyphen, but it can be changed to something else if desired. The -main function just makes a couple of calls to the binomial function to assure that we are getting the correct results. To do that, compile the class and run the program.
C:\projects\com.domain.tiny>lein run
You should see output shown in the original answer.
Now package it up in a jar and put it someplace convenient. Copy the Clojure jar there too.
C:\projects\com.domain.tiny>lein jar
Created C:\projects\com.domain.tiny\target\com.domain.tiny-0.1.0-SNAPSHOT.jar
C:\projects\com.domain.tiny>mkdir \target\lib
C:\projects\com.domain.tiny>copy target\com.domain.tiny-0.1.0-SNAPSHOT.jar target\lib\
1 file(s) copied.
C:\projects\com.domain.tiny>copy "C:<path to clojure jar>\clojure-1.5.1.jar" target\lib\
1 file(s) copied.
The Java Part
Leiningen has a built-in task, lein-javac, that should be able to help with the Java compilation. Unfortunately, it seems to be broken in version 2.1.3. It can't find the installed JDK and it can't find the Maven repository. The paths to both have embedded spaces on my system. I assume that is the problem. Any Java IDE could handle the compilation and packaging too. But for this post, we're going old school and doing it at the command line.
First create the file Main.java with the contents shown in the original answer.
To compile java part
javac -g -cp target\com.domain.tiny-0.1.0-SNAPSHOT.jar -d target\src\com\domain\Main.java
Now create a file with some meta-information to add to the jar we want to build. In Manifest.txt, add the following text
Class-Path: lib\com.domain.tiny-0.1.0-SNAPSHOT.jar lib\clojure-1.5.1.jar
Main-Class: Main
Now package it all up into one big jar file, including our Clojure program and the Clojure jar.
C:\projects\com.domain.tiny\target>jar cfm Interop.jar Manifest.txt Main.class lib\com.domain.tiny-0.1.0-SNAPSHOT.jar lib\clojure-1.5.1.jar
To run the program:
C:\projects\com.domain.tiny\target>java -jar Interop.jar
(binomial 5 3): 10.0
(binomial 10042, 111): 4.9068389575068143E263
The output is essentially identical to that produced by Clojure alone, but the result has been converted to a Java double.
As mentioned, a Java IDE will probably take care of the messy compilation arguments and the packaging.
As of Clojure 1.6.0, there is a new preferred way to load and invoke Clojure functions. This method is now preferred to calling RT directly (and supersedes many of the other answers here). The javadoc is here - the main entry point is clojure.java.api.Clojure.
To lookup and call a Clojure function:
IFn plus = Clojure.var("clojure.core", "+");
plus.invoke(1, 2);
Functions in clojure.core are automatically loaded. Other namespaces can be loaded via require:
IFn require = Clojure.var("clojure.core", "require");
require.invoke(Clojure.read("clojure.set"));
IFns can be passed to higher order functions, e.g. the example below passes plus to read:
IFn map = Clojure.var("clojure.core", "map");
IFn inc = Clojure.var("clojure.core", "inc");
map.invoke(inc, Clojure.read("[1 2 3]"));
Most IFns in Clojure refer to functions. A few, however, refer to non-function data values. To access these, use deref instead of fn:
IFn printLength = Clojure.var("clojure.core", "*print-length*");
IFn deref = Clojure.var("clojure.core", "deref");
deref.invoke(printLength);
Sometimes (if using some other part of the Clojure runtime), you may need to ensure that the Clojure runtime is properly initialized - calling a method on the Clojure class is sufficient for this purpose. If you do not need to call a method on Clojure, then simply causing the class to load is sufficient (in the past there has been a similar recommendation to load the RT class; this is now preferred):
Class.forName("clojure.java.api.Clojure")
EDIT This answer was written in 2010, and worked at that time. See Alex Miller's answer for more modern solution.
What kind of code are calling from Java? If you have class generated with gen-class, then simply call it. If you want to call function from script, then look to following example.
If you want to evaluate code from string, inside Java, then you can use following code:
import clojure.lang.RT;
import clojure.lang.Var;
import clojure.lang.Compiler;
import java.io.StringReader;
public class Foo {
public static void main(String[] args) throws Exception {
// Load the Clojure script -- as a side effect this initializes the runtime.
String str = "(ns user) (defn foo [a b] (str a \" \" b))";
//RT.loadResourceScript("foo.clj");
Compiler.load(new StringReader(str));
// Get a reference to the foo function.
Var foo = RT.var("user", "foo");
// Call it!
Object result = foo.invoke("Hi", "there");
System.out.println(result);
}
}
EDIT: I wrote this answer almost three years ago. In Clojure 1.6 there is a proper API exactly for the purpose of calling Clojure from Java. Please Alex Miller's answer for up to date information.
Original answer from 2011:
As I see it, the simplest way (if you don't generate a class with AOT compilation) is to use clojure.lang.RT to access functions in clojure. With it you can mimic what you would have done in Clojure (no need to compile things in special ways):
;; Example usage of the "bar-fn" function from the "foo.ns" namespace from Clojure
(require 'foo.ns)
(foo.ns/bar-fn 1 2 3)
And in Java:
// Example usage of the "bar-fn" function from the "foo.ns" namespace from Java
import clojure.lang.RT;
import clojure.lang.Symbol;
...
RT.var("clojure.core", "require").invoke(Symbol.intern("foo.ns"));
RT.var("foo.ns", "bar-fn").invoke(1, 2, 3);
It is a bit more verbose in Java, but I hope it's clear that the pieces of code are equivalent.
This should work as long as Clojure and the source files (or compiled files) of your Clojure code is on the classpath.
I agree with clartaq's answer, but I felt that beginners could also use:
step-by-step information on how to actually get this running
information that's current for Clojure 1.3 and recent versions of leiningen.
a Clojure jar that also includes a main function, so it can be run standalone or linked as a library.
So I covered all that in this blog post.
The Clojure code looks like this:
(ns ThingOne.core
(:gen-class
:methods [#^{:static true} [foo [int] void]]))
(defn -foo [i] (println "Hello from Clojure. My input was " i))
(defn -main [] (println "Hello from Clojure -main." ))
The leiningen 1.7.1 project setup looks like this:
(defproject ThingOne "1.0.0-SNAPSHOT"
:description "Hello, Clojure"
:dependencies [[org.clojure/clojure "1.3.0"]]
:aot [ThingOne.core]
:main ThingOne.core)
The Java code looks like this:
import ThingOne.*;
class HelloJava {
public static void main(String[] args) {
System.out.println("Hello from Java!");
core.foo (12345);
}
}
Or you can also get all the code from this project on github.
This works with Clojure 1.5.0:
public class CljTest {
public static Object evalClj(String a) {
return clojure.lang.Compiler.load(new java.io.StringReader(a));
}
public static void main(String[] args) {
new clojure.lang.RT(); // needed since 1.5.0
System.out.println(evalClj("(+ 1 2)"));
}
}
If the use case is to include a JAR built with Clojure in a Java application, I have found having a separate namespace for the interface between the two worlds to be beneficial:
(ns example-app.interop
(:require [example-app.core :as core])
;; This example covers two-way communication: the Clojure library
;; relies on the wrapping Java app for some functionality (through
;; an interface that the Clojure library provides and the Java app
;; implements) and the Java app calls the Clojure library to perform
;; work. The latter case is covered by a class provided by the Clojure lib.
;;
;; This namespace should be AOT compiled.
;; The interface that the java app can implement
(gen-interface
:name com.example.WeatherForecast
:methods [[getTemperature [] Double]])
;; The class that the java app instantiates
(gen-class
:name com.example.HighTemperatureMailer
:state state
:init init
;; Dependency injection - take an instance of the previously defined
;; interface as a constructor argument
:constructors {[com.example.WeatherForecast] []}
:methods [[sendMails [] void]])
(defn -init [weather-forecast]
[[] {:weather-forecast weather-forecast}])
;; The actual work is done in the core namespace
(defn -sendMails
[this]
(core/send-mails (.state this)))
The core namespace can use the injected instance to accomplish its tasks:
(ns example-app.core)
(defn send-mails
[{:keys [weather-forecast]}]
(let [temp (.getTemperature weather-forecast)] ...))
For testing purposes, the interface can be stubbed:
(example-app.core/send-mails
(reify com.example.WeatherForecast (getTemperature [this] ...)))
Other technique that works also with other languages on top of JVM is to declare an interface for functions you want to call and then use 'proxy' function to create instance that implemennts them.
You can also use AOT compilation to create class files representing your clojure code. Read the documentation about compilation, gen-class and friends in the Clojure API docs for the details about how to do this, but in essence you will create a class that calls clojure functions for each method invocation.
Another alternative is to use the new defprotocol and deftype functionality, which will also require AOT compilation but provide better performance. I don't know the details of how to do this yet, but a question on the mailing list would probably do the trick.