Java REPL (Read-Eval-Print-Loop): previously called Kulla.
The JShell API can be used via applications as well.
Windows Users
Check if JAVA_HOME is set on your machine
c:\... echo %JAVA_HOME%
"c:\Program Files\Java\jdk-9\"
Look for the JShell REPL in the JAVA_HOME\bin folder:
c:\... dir /W %JAVA_HOME%\bin\js*
Directory of C:\Program Files\Java\jdk-9\bin
jshell.exe jsound.dll jsoundds.dll jstack.exe jstat.exe
jstatd.exe
And locate the command from the above list. Ensure JAVA_HOME\bin is in your PATH.
c:\... echo %PATH%
c:\Program Files\Java\jdk-9\bin\;....
If JAVA_HOME is not set on your Windows machine (or it's not set to Java 9) or PATH doesn't show the bin folder for JDK-9, You should set the correct value by following these steps:
- Invoke the Control Panel, and then System.
- Click Advanced and then Environment Variables.
- Add the location of the JDK-9 folder to the
JAVA_HOME, - and add or edit the location of the bin folder of the JDK installation to
PATHvariable in System Variables.
You should set the JAVA_HOME value between quotation marks.
Once JAVA_HOME and PATH are set, you must close the current console and then open a new Windows Console. Now you are ready to start JShell with the following command:
c:\... jshell
MacOs / Linux Users
Look for the JShell REPL in the JAVA_HOME/bin folder:
$ ls $JAVA_HOME/bin/
appletviewer java javapackager jdb jinfo jps jstatd policytool serialver wsimport
idlj javac javaws jdeprscan jjs jrunscript jweblauncher rmic servertool xjc
jaotc javadoc jcmd jdeps jlink [jshell] keytool rmid tnameserv
jar javah jconsole jhsdb jmap jstack orbd rmiregistry unpack200
jarsigner javap jcontrol jimage jmod jstat pack200 schemagen wsgen
And locate the command from the above list. Ensure JAVA_HOME/bin is in your PATH.
$ echo $PATH
/usr/lib/jvm/jdk-9/bin/:....
If JAVA_HOME is not set on your Mac or Linux machine (or it's not set to Java 9) or PATH doesn't show the bin folder for JDK-9, You should set the correct values by following the next steps:
(For Mac Users the JDK 9 path could be /Library/Java/JavaVirtualMachines/jdk9/Contents/Home)
$ export JAVA_HOME=/usr/lib/jvm/jdk-9/
$ export PATH=/usr/lib/jvm/jdk-9/bin:$PATH
Once JAVA_HOME and PATH are set you can start JShell with the next command
$ jshell
After you start JShell (Windows, MacOs and Linux Users) you get taken to the JShell prompt:
| Welcome to JShell -- Version 9-ea
| For an introduction type: /help intro
jshell>
The first command to try in the REPL:
jshell> /?
Let's assign a string to a variable and see its effect:
jshell> String name = "ICELAND-JUG"
name ==> "ICELAND-JUG"
Define a void method and execute it:
jshell> void hello() { System.out.println("Hi " + name); }
| modified method hello()
jshell> hello()
Hi ICELAND-JUG
jshell> hello();
Hi ICELAND-JUG
As you can see there is no need to use a semi-colon at the end of single line commands in the REPL.
By default, the REPL loads with pre-defined imports:
jshell> /i
| import java.io.*
| import java.math.*
| import java.net.*
| import java.nio.file.*
| import java.util.*
| import java.util.concurrent.*
| import java.util.function.*
| import java.util.prefs.*
| import java.util.regex.*
| import java.util.stream.*
Now create your own variables, method, etc.. and play with them in the REPL.
Try using the /v or the /m REPL commands, here's a sample output:
jshell> /v
| String name = "ICELAND-JUG"
| int i = 10
| int j = 15
| int k = 25
jshell> /m
| void hello()
Now try some more REPL commands, at the jshell prompt:
/vars
/list
/methods
/edit hello
/imports
As mentioned earlier, Java 9 provides a JShell API that we can access from within our application:
JShell shell = JShell.create();
shell.eval("int x = 5;");
Stream<VarSnippet> vars = shell.variables();The above block is not for the REPL.
Some more Java 9 improvements to try in JShell (thanks to @sjmaple for the contribution)
jshell> import java.time.*;
jshell> void printf(Object a) {
System.out.println(a);
}
| created method printf(Object)
Collections of factory methods:
1-10 arguments + varargs:
jshell> List.of(1, 2, 3, 4, 5)
$3 ==> [1, 2, 3, 4, 5]
jshell> Map.of("hello", "world", "status", "mad")
$4 ==> {hello=world, status=mad}
jshell> Set.of(1,2,4)
$5 ==> [2, 4, 1]
jshell> Set.of(1,2,4) <=== no change in order of the elements
$6 ==> [2, 4, 1]
Implementation classes 2 elements vs N elements:
jshell> List.of(1).getClass()
$9 ==> class java.util.ImmutableCollections$List1
jshell> List.of(1, 2).getClass()
$10 ==> class java.util.ImmutableCollections$List2
Streams:
- dropwhile/takewhile:
jshell> IntStream.range(1, 10).dropWhile(x -> x < 5).forEach(System.out::println)
5
6
7
8
9
- iterate
jshell> IntStream.iterate(0, x -> x + 1).filter(x -> x < 10).forEach(System.out::println)
0
1
2
3
4
5
6
7
8
9
Press Ctrl-C to get back to the jshell prompt
- Stream from Optional:
jshell> Optional.of(1).stream().map(x -> x*3);
$16 ==> java.util.stream.ReferencePipeline$3@4e7dc304
jshell> Optional.empty().or(() -> Optional.of("Hello"))
$17 ==> Optional[Hello]
jshell> Optional.empty().ifPresent(null)
jshell> Optional.empty().ifPresentOrElse(null, () -> printf("hello"));
hello
- Date and time improvements:
jshell> import java.time.*;
jshell> LocalDate.now()
$21 ==> 2017-08-15
jshell> $21.datesUntil(LocalDate.of(2017, 8, 20)).forEach(System.out::println)
2017-08-15
2017-08-16
2017-08-17
2017-08-18
2017-08-19
Concurrency
- Completable futures:
jshell> CompletableFuture<String> future = new CompletableFuture<>()
future ==> java.util.concurrent.CompletableFuture@77846d2c[Not completed]
jshell> future.copy()
$28 ==> java.util.concurrent.CompletableFuture@16f7c8c1[Not completed]
jshell> $28.complete("Hello world")
$29 ==> true
jshell> $29.get()
| Error:
| boolean cannot be dereferenced
| $29.get()
| ^-----^
jshell> future.isDone()
$30 ==> false
- delays / timeouts:
* completeOnTimeout(T value, long timeout, TimeUnit unit)
jshell> future.completeOnTimeout("Isn't this amazing", 1, TimeUnit.SECONDS)
$31 ==> java.util.concurrent.CompletableFuture@281e3708[Not completed, 2 dependents]
* orTimeout(1, TimeUnit.SECONDS)
jshell> future.orTimeout(1, TimeUnit.SECONDS)
$32 ==> java.util.concurrent.CompletableFuture@281e3708[Completed normally]
HttpClient
Run jshell with a CLI argument to load a JDK module (jdk.incubator.httpclient)
$ jshell --add-modules jdk.incubator.httpclient
jshell> import jdk.incubator.http.*;
jshell> String body = HttpClient.newHttpClient().send(HttpRequest.newBuilder(URI.create("https://www.google.com")).GET().build(), HttpResponse.BodyHandler.asString()).body();
Try using http in the URL address? What is the expected result?
jshell> String body = HttpClient.newHttpClient().send(HttpRequest.newBuilder(URI.create("http://www.google.com")).GET().build(), HttpResponse.BodyHandler.asString()).body();
body ==> "<HTML><HEAD><meta http-equiv=\"content-type\" co ... A>.\r\n</BODY></HTML>\r\n"
Try without https in the URL address? What is the expected result?
java.lang.NullPointerException
Now that we can make http calls via the REPL, lets have a look at how that works. We can use Java 9's new API to create Http calls. Check the JavaDoc for HttpRequest or use the example above.
Make 50 http GET calls to http://www.google.com via the REPL.
Process management API:
Reload jshell using the usual CLI invocation:
$ jshell
jshell> ProcessHandle currentProcess = ProcessHandle.current();
currentProcess ==> 10420
jshell> System.out.println("Current Process Id: = " + currentProcess.pid());
Current Process Id: = 10420
- ProcessHandle.Info info()
- Optional<String[]> arguments()
- Optional command()
- Optional commandLine()
- Optional startInstant()
- Optional totalCpuDuration()
- Optional user()
jshell> currentProcess.info()
$5 ==> [user: Optional[username], cmd: /usr/lib/jvm/jdk-9/bin/java, args: [-agentlib:jdwp=transport=dt_socket,address=localhost:40499, jdk.jshell.execution.RemoteExecutionControl, 41667], startTime: Optional[2017-08-15T17:36:30.240Z], totalTime: Optional[PT0.88S]]
- pid, info, children processes, destroy processes:
jshell> ProcessHandle currentProcess = ProcessHandle.current();
currentProcess ==> 10420
jshell> currentProcess.pid()
$7 ==> 10420
jshell> ProcessHandle.of(10420)
$9 ==> Optional[10420]
jshell> $9.get()
$10 ==> 10420
jshell> $10.info()
$11 ==> [user: Optional[username], cmd: /usr/lib/jvm/jdk-9/bin/java, args: [-agentlib:jdwp=transport=dt_socket,address=localhost:40499, jdk.jshell.execution.RemoteExecutionControl, 41667], startTime: Optional[2017-08-15T17:36:30.240Z], totalTime: Optional[PT1.09S]]
jshell> Stream<ProcessHandle> childProc = ProcessHandle.current().children();
childProc ==> java.util.stream.ReferencePipeline$2@21213b92
jshell> Process process = Runtime.getRuntime().exec ("/bin/bash");
process ==> Process[pid=709, exitValue="not exited"]
jshell> childProc.count()
$47 ==> 1
jshell> Stream<ProcessHandle> childProc = ProcessHandle.current().children();
childProc ==> java.util.stream.ReferencePipeline$2@10d59286
jshell> childProc.forEach(procHandle -> { System.out.println(procHandle.destroy() ? "Could not kill process " + procHandle.pid() : "Terminated " + procHandle.pid()); });
Could not kill process 709
or
Terminated 709
or
| java.lang.IllegalStateException thrown: stream has already been operated upon or closed
| at AbstractPipeline.evaluate (AbstractPipeline.java:229)
| at ReferencePipeline.forEach (ReferencePipeline.java:430)
| at (#62:1)
StackWalker:
jshell> StackWalker.getInstance().walk(s -> s.limit(5).collect(Collectors.toList()));
[do_it$(java:15), jdk.jshell/jdk.jshell.execution.DirectExecutionControl.invoke(DirectExecutionControl.java:209), jdk.jshell/jdk.jshell.execution.RemoteExecutionControl.invoke(RemoteExecutionControl.java:116), jdk.jshell/jdk.jshell.execution.DirectExecutionControl.invoke(DirectExecutionControl.java:119), jdk.jshell/jdk.jshell.execution.ExecutionControlForwarder.processCommand(ExecutionControlForwarder.java:134)]
Stop doing new Exception when you need to inspect the stack while in the REPL.
JDK structure changes
- Semantic versioning:
jshell> Runtime.version()
$13 ==> 9-ea+170-jigsaw-nightly-h6410-20170518
jshell> $13.major()
$14 ==> 9
jshell> $13.minor()
$15 ==> 0
Language improvements continue (Project Coin):
_ is an invalid identifier (for lambdas)
jshell> int _ = 1
| Error:
| as of release 9, '_' is a keyword, and may not be used as an identifier
| int _ = 1
| ^
| Error:
| reached end of file while parsing
| int _ = 1
| ^
jshell> int __ = 1
__ ==> 1
private methods in interfaces:
jshell> interface A { private int zero() { return 0;} default int one() { return zero() + 1;}}
| created interface A
effectively final in try with resources:
jshell> boolean a() throws Exception { Socket s = new Socket(); return s.isClosed(); }
| created method a()
jshell> boolean a() throws Exception { Socket s = new Socket(); try (s) { } return s.isClosed(); }
| modified method a()
UTF-8 property files:
Create a text file call properties.properties containing, save the file and load jshell:
# This is the default properties.properties file
s1 = computer
s2 = disk
s3 = monitor
s4 = keyboard
jshell> new PropertyResourceBundle(new FileInputStream("properties.properties")).handleGetObject("s2")
$1 ==> "computer"
Finally, exit the REPL via one of the two ways:
jshell> /exit
| Goodbye
Or
jshell> [Ctrl][D]
(quietly exits)
Read all about the Java REPL at
- Project Kulla: https://adoptopenjdk.gitbooks.io/adoptopenjdk-getting-started-kit/content/en/openjdk-projects/kulla/kulla.html
Follow Hendrik's hands-on presentation on JShell to learn about intermediate and advanced features: https://youtu.be/y868lMk6NtY?t=13156
Try some more JShell examples:
- https://zeroturnaround.com/rebellabs/the-best-java-9-language-and-api-improvements/
- http://jakubdziworski.github.io/java/2016/07/31/jshell-getting-started-examples.html
- REPL_Tutorial.pdf - https://community.oracle.com/docs/DOC-1016569
- https://www.infoq.com/articles/Java9-New-HTTP-2-and-REPL (see towards the bottom of the blog post)
Some online Java REPLs to play with:
- http://www.javarepl.com/term.html
- https://github.com/albertlatacz/java-repl
- https://repl.it/languages/java
- https://teamtreehouse.com/community/installing-java-repl
Credits
- Contributions from Hendrik Ebbers (@hendrikebbers) has been distilled above - thanks @hendrikebbers