Conferences are a great place to meet Java luminaries. Devoxx France was one such opportunity to meet Java language architect, ex-colleague and an old friend – Brian Goetz (@briangoetz). We talked about JDK 9 and he was all raving about REPL. He mentioned that even though there are a lot of significant features, such as modularity and HTTP2 client, in Java SE 9, but this tool is going to be talked about the most often. The statement makes sense since it will really simplify exploration of Java APIs, prototyping, demos in conferences, and similar tasks a lot simpler. This blog is coming out of our discussion there and his strong vote on REPL!
Read-Evaluate-Print-Loop has been there in Lisp, Python, Ruby, Groovy, Clojure, and other languages for a while. Unix shell is a REPL which can read shell commands, evaluate them, print the output, and goes back in the loop to do the same thing.
You can read all about REPL in JDK 9 in JEP 222. Summary from the JEP is:
Provide an interactive tool which evaluates declarations, statements, and expressions of the Java Programming Language: that is, provide a Read-Evaluate-Print Loop (REPL) for the Java Programming Language. Also, provide an API on which the tool is built, enabling external tools to supply this functionality. JEP 222
The motivation is also clearly spelt out in the JEP:
Without the ceremony of class Foo { public static void main(String[] args) { … } }, learning and exploration is streamlined. JEP 222
JEP 222 targets to ship REPL with JDK 9 but openjdk.java.net/projects/jdk9 does not list it is as “targeted” or “proposed to target”. Seems like a documentation bug :)
As of JDK 9 build 61, REPL is not integrated, and needs to be built separately. Eventually, at some time before JDK 9 is released, this tool will be integrated in the build.
Lets see what does it require to have it running on OSX. This blog followed Java 9 REPL – Getting Started Guide to build and run REPL. In addition, it provides complete log output from the commands which might be helpful for some.
Lets get started!
Install JDK 9
- Download the latest build, 61 at the time of this writing.
- Setup JAVA_HOME as:1
export JAVA_HOME=`/usr/libexec/java_home -v1.9`More details on setting JAVA_HOME on OSX are here.
- Verify the version:1234
~> java -versionjava version"1.9.0-ea"Java(TM) SE Runtime Environment (build1.9.0-ea-b61)Java HotSpot(TM)64-Bit Server VM (build1.9.0-ea-b61, mixed mode)
Checkout and Install jline2
jline2 is a Java library for handling console input. Check it out:
1 2 3 4 5 6 7 | workspaces> git clone git://github.com/jline/jline2.gitCloning into 'jline2'...remote: Counting objects: 6419, done.remote: Total 6419 (delta 0), reused 0 (delta 0), pack-reused 6419Receiving objects: 100% (6419/6419), 3.23 MiB | 80.00 KiB/s, done.Resolving deltas: 100% (2945/2945), done.Checking connectivity... done. |
And then build it:
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 | jline2> mvn install[INFO] Scanning for projects...Downloading: https://repo.maven.apache.org/maven2/org/apache/maven/scm/maven-scm-provider-gitexe/1.8.1/maven-scm-provider-gitexe-1.8.1.pomDownloaded: https://repo.maven.apache.org/maven2/org/apache/maven/scm/maven-scm-provider-gitexe/1.8.1/maven-scm-provider-gitexe-1.8.1.pom (3 KB at 0.3 KB/sec). . .[INFO] Installing /Users/arungupta/workspaces/jline2/target/jline-2.13-SNAPSHOT-sources.jar to /Users/arungupta/.m2/repository/jline/jline/2.13-SNAPSHOT/jline-2.13-SNAPSHOT-sources.jar[INFO] Installing /Users/arungupta/workspaces/jline2/target/jline-2.13-SNAPSHOT-tests.jar to /Users/arungupta/.m2/repository/jline/jline/2.13-SNAPSHOT/jline-2.13-SNAPSHOT-tests.jar[INFO] ------------------------------------------------------------------------[INFO] BUILD SUCCESS[INFO] ------------------------------------------------------------------------[INFO] Total time: 09:52 min[INFO] Finished at: 2015-05-06T19:23:14-07:00[INFO] Final Memory: 25M/146M[INFO] ------------------------------------------------------------------------ |
Clone and Build JDK 9 REPL
OpenJDK codename for the project is Kulla which means “The God of Builders”. Planned name for the tool is jshell.
- Check out the workspace:12345678
workspaces> hg clone http://hg.openjdk.java.net/kulla/dev kullarequesting all changesadding changesetsadding manifestsadding file changesadded1453changesets with2226changes to172filesupdating to branchdefault109files updated,0files merged,0files removed,0files unresolved - Get the sources:01020304050607080910111213141516171819202122232425262728293031323334353637383940414243444546474849505152535455565758596061626364656667686970717273747576777879808182838485868788899091929394
workspaces> cd kullakulla> chmod +x ./get_source.shkulla> ./get_source.sh# Repositories: corba jaxp jaxws langtools jdk hotspot nashorncorba: hg clone http://hg.openjdk.java.net/kulla/dev/corba corbajaxp: hg clone http://hg.openjdk.java.net/kulla/dev/jaxp jaxpcorba: requesting all changesjaxp: requesting all changescorba: adding changesetsjaxp: adding changesetscorba: adding manifestsjaxp: adding manifestscorba: adding file changesjaxp: adding file changescorba: added669changesets with4913changes to2576filescorba: updating to branchdefaultcorba:1184files updated,0files merged,0files removed,0files unresolvedjaxws: hg clone http://hg.openjdk.java.net/kulla/dev/jaxws jaxwsjaxws: requesting all changesjaxws: adding changesetsjaxws: adding manifestsjaxws: adding file changesjaxp: added728changesets with10192changes to7393filesjaxp: updating to branchdefaultjaxp:3044files updated,0files merged,0files removed,0files unresolvedlangtools: hg clone http://hg.openjdk.java.net/kulla/dev/langtools langtoolslangtools: requesting all changeslangtools: adding changesetslangtools: adding manifestsjaxws: added589changesets with20521changes to10746filesjaxws: updating to branchdefaultjaxws:3750files updated,0files merged,0files removed,0files unresolvedjdk: hg clone http://hg.openjdk.java.net/kulla/dev/jdk jdklangtools: adding file changesjdk: requesting all changesjdk: adding changesetsjdk: adding manifestslangtools: added3173changesets with28112changes to9103fileslangtools: updating to branchdefaultlangtools:7100files updated,0files merged,0files removed,0files unresolvedhotspot: hg clone http://hg.openjdk.java.net/kulla/dev/hotspot hotspothotspot: requesting all changeshotspot: adding changesetshotspot: adding manifestshotspot: adding file changeshotspot: added8073changesets with45889changes to6290fileshotspot: updating to branchdefaulthotspot:5030files updated,0files merged,0files removed,0files unresolvednashorn: hg clone http://hg.openjdk.java.net/kulla/dev/nashorn nashornnashorn: requesting all changesnashorn: adding changesetsnashorn: adding manifestsjdk: adding file changesnashorn: adding file changesnashorn: added1252changesets with11596changes to3595filesnashorn: updating to branchdefaultnashorn:2867files updated,0files merged,0files removed,0files unresolvedjdk: added11805changesets with116593changes to42135filesjdk: updating to branchdefaultjdk:23192files updated,0files merged,0files removed,0files unresolved# Repositories: . corba jaxp jaxws langtools jdk hotspot nashorn.: cd . && hg pull -ucorba: cd corba && hg pull -ujaxp: cd jaxp && hg pull -ujaxws: cd jaxws && hg pull -ulangtools: cd langtools && hg pull -ujdk: cd jdk && hg pull -uhotspot: cd hotspot && hg pull -unashorn: cd nashorn && hg pull -ujaxws: pulling from http://hg.openjdk.java.net/kulla/dev/jaxwscorba: pulling from http://hg.openjdk.java.net/kulla/dev/corbalangtools: pulling from http://hg.openjdk.java.net/kulla/dev/langtoolshotspot: pulling from http://hg.openjdk.java.net/kulla/dev/hotspotjdk: pulling from http://hg.openjdk.java.net/kulla/dev/jdk.: pulling from http://hg.openjdk.java.net/kulla/devjaxp: pulling from http://hg.openjdk.java.net/kulla/dev/jaxpnashorn: pulling from http://hg.openjdk.java.net/kulla/dev/nashornnashorn: searchingforchangesnashorn: no changes foundjaxws: searchingforchangesjaxws: no changes foundjaxp: searchingforchangesjaxp: no changes foundcorba: searchingforchangescorba: no changes foundjdk: searchingforchangeshotspot: searchingforchangeshotspot: no changes foundjdk: no changes found.: searchingforchanges.: no changes foundlangtools: searchingforchangeslangtools: no changes found - Edit
langtools/repl/scripts/compile.shscript such that it looks like:1234#!/bin/shJLINE2LIB=/Users/arungupta/workspaces/jline2/target/jline-2.13-SNAPSHOT.jarmkdir -p buildjavac -Xlint:unchecked -Xdiags:verbose -cp ${JLINE2LIB} -d build ../src/jdk.jshell/share/classes/*/*/*.java ../src/jdk.jshell/share/classes/*/*/*/*/*.java ../src/jdk.jshell/share/classes/*/*/*/*/*/*.javaNotice, the only edits are
#!/bin/shfor OSX and addingJLINE2LIBto the location of your previously compiledjline2workspace.javacis picked from JAVA_HOME that is referring to JDK 9. - Compile the REPL tool by invoking the script from
langtools/repldirectory:12repl> chmod +x ./scripts/compile.shrepl> ./scripts/compile.sh
Run JDK 9 REPL
- Edit
langtools/repl/scripts/run.shscript such that it looks like:123#!/bin/shJLINE2LIB=/Users/arungupta/workspaces/jline2/target/jline-2.13-SNAPSHOT.jarjava -ea -esa -cp build:${JLINE2LIB} jdk.internal.jshell.tool.JShellTool"$@"Notice, the only edits are
!/bin/shfor OSX and addingJLINE2LIB. - Run REPL as:123
repl> ./scripts/run.sh| Welcome to JShell -- Version0.428| Type /helpforhelp
JDK 9 REPL Hello World
Unlike the introduction of bouncing ball or dancing Duke that was used to introduce Java, we’ll just use the conventional Hello World for REPL
Run “Hello World” as:
1 2 | -> System.out.println("Hello World");Hello World |
Voila!
No public static void main, no class creation, no ceremony, just clean and simple Java code. The entered text is called as “snippet”.
The complete Java code can be seen using /list all and looks like:
01 02 03 04 05 06 07 08 09 10 11 | -> /list all 1 : import java.util.*; 2 : import java.io.*; 3 : import java.math.*; 4 : import java.net.*; 5 : import java.util.concurrent.*; 6 : import java.util.prefs.*; 7 : import java.util.regex.*; 8 : void printf(String format, Object... args) { System.out.printf(format, args); } 9 : System.out.println("Hello World"); |
This snippet can be saved to a file as:
1 | -> /save test |
Note this is not a Java file. Saved snippet is exactly what was entered:
1 2 | repl> more testSystem.out.println("Hello World"); |
And the tool can be exited as:
1 2 | -> /exit| Goodbye |
Or you can just hit Ctrl+C.
Complete list of commands can be easily seen:
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 | -> /helpType a Java language expression, statement, or declaration.Or type one of the following commands:/l or /list [all] -- list the source you have typed /seteditor <executable> -- set the external editor command to use/e or /edit <name or id> -- edit a source entry referenced by name or id/- or /drop <name or id> -- delete a source entry referenced by name or id/s or /save [all|history] <file> -- save the source you have typed/o or /open <file> -- open a file as source input/v or /vars -- list the declared variables and their values/m or /methods -- list the declared methods and their signatures/c or /classes -- list the declared classes/x or /exit -- exit the REPL/r or /reset -- reset everything in the REPL/f or /feedback <level> -- feedback information: off, concise, normal, verbose, default, or ?/p or /prompt -- toggle display of a prompt/cp or /classpath <path> -- add a path to the classpath/h or /history -- history of what you have typed /setstart <file> -- read file and set as the new start-up definitions /savestart <file> -- save the default start-up definitions to the file/? or /help -- this help messageSupported shortcuts include:<tab> -- show possible completions for the current textShift-<tab> -- for current method or constructor invocation, show a synopsis of the method/constructor |
JDK 9 REPL Next Steps and Feedback
Follow the REPL Tutorial to learn more about the tool’s capability. Here is a quick overview: 
- Accepts Java statements, variable, method, and class definitions, imports, and expressions
- Commands for settings and to display information, such as
/listto display the list of snippets,/varsto display the list of variables,/saveto save your snippets,/opento read them back in. - History of snippets is available, snippets can be edited by number, and much more
Here is an RFE that would be useful:
- Export a snippet as full blown Java class
A subsequent blog will showcase how this could be used for playing with a Java EE application. How would you use REPL?
Discuss the project/issues on kulla-dev.
Enjoy!
| Reference: | JDK 9 REPL: Getting Started from our JCG partner Arun Gupta at the Miles to go 2.0 … blog. |
