Hello World

Generate the lexer

Simples example usage of the jflex() rule:

jflex(
    name = "gen_hello_lexer",
    srcs = ["helloworld.flex"],
    outputs = ["HelloWorld.java"],
    visibility = ["//visibility:private"],
)

Invoke this target:

blaze build //java/jflex/examples/helloworld:gen_hello_lexer

Tip The Bazel commands work from any directory within the workspace.

Expected output:

(examples/)bazel-genfiles/java/jflex/examples/helloworld/Yylex.java Java code generated from the flex file.

The output (generated Java sources) of the jflex() rule can be reused in other targets, for instance as the source of a java_library():

java_library(
    name = "helloworld",
    srcs = [":gen_hello_lexer"],
)

Generate the Java library

blaze build //java/jflex/examples/helloworld:helloworld

Tip: Since “:helloworld” depends on “:gen_hello_lexer”, you don’t need to invoke the previous target manually. This is true for any target.

Tip Since the target name is the same as the last part of the path, you can use the short form //java/jflex/examples/helloworld.

Expected output:

(examples/)bazel-bin/java/jflex/examples/helloworld/libhelloworld.jar The Java archive containing the HelloWorld class

From a java_library(), it’s easy to build an executable with the java_binary() rule:

java_binary(
    name = "helloworld_bin",
    main_class = "jflex.examples.helloworld.HelloWorld",
    runtime_deps = [":helloworld"],
)

To build the executable:

bazel build //java/jflex/examples/helloworld:helloworld_bin

To run this sample program:

bazel run //java/jflex/examples/helloworld:helloworld_bin

Everything works, but the program complains that no input file was provided:

Usage : java HelloWorld [ –encoding ] <inputfile(s)>

We provide a sample input file in javatest/flex/examples/helloworld/data/test.txt with the following content:

hello
name John Doe
hello

You can pass arguments to the program after --. Hence you can invoke:

bazel run //java/jflex/examples/helloworld:helloworld_bin -- /full/path/to/javatests/jflex/examples/helloworld/testdata.txt

You should see

Hello World! Hello John Doe!

N.B. Relative paths don’t work in bazel run.

Alternatively, you can use the generated artifact. From the examples directory:

bazel-bin/java/jflex/examples/helloworld/helloworld_bin javatests/jflex/examples/helloworld/testdata.txt

To execute the tests

blaze test //javatests/jflex/examples/helloworld/...

In this sample, there is only one test:

The example:

Creates an in-memory OutputStream
Redirects System.out to this in-memory stream
Calls the main method of the HelloWorld lexer
Compares line by line the content of the in-memory output with a hard-coded string that contains the expected output.