CODEWITHSUNDEEP
javajava-9java-modulejava-platform-module-system
Anton Balaniuc
Anton Balaniuc

Reputation: 11739

Why Java Automatic modules do not contain exports section

There is a mechanism to convert regular jar into an automatic module starting from , by simple placing it onto modulepath instead of classpath. An automatic module has the following properties:

An automatic module is a named module that is defined implicitly, since it does not have a module declaration. Every package in an automatic module is, therefore, considered to be exported even if it might actually be intended only for internal use.

So all packages inside an automatic module are available. But if we inspect an automatic module with --describe-module directive there is no exports section in the output.

For example an automatic module main

jar --file main.jar --describe-module
com.foo jar:file:///.../code/module/main.jar/!module-info.class
requires java.base mandated
contains com.foo

Why is there no exports section if package com.foo is considered to be exported anyway? I find this a bit confusing: --describe-module suggests that there are no exported packages, but at the same time, main is an automatic module so everything is exported implicitly.

Upvotes: 3

Views: 945

Answers (1)

Naman
Naman

Reputation: 31868

I believe, the command line option --describe-module within the jar tool is just to describe what the jar file is about, if its an explicit module the complete descriptor is shared whie for automatic module just the name is depicted.

Quoting from the command jar --help

Print the module descriptor, or automatic module name

On the other hand, if you try using jdeps for generating module-info.java of an automatic module given a jar file, you can notice that such packages exports are actually present in the module declaration. Say for example:

jdeps -verbose:class --generate-module-info ../Desktop  ~/.m2/repository/org/apache/commons/commons-lang3/3.8.1/commons-lang3-3.8.1.jar

writes to .../Desktop/org.apache.commons.lang3/module-info.java the following 

module org.apache.commons.lang3 {
    requires transitive java.desktop;

    exports org.apache.commons.lang3;
    exports org.apache.commons.lang3.arch;
    exports org.apache.commons.lang3.builder;
    exports org.apache.commons.lang3.concurrent;
    exports org.apache.commons.lang3.event;
    exports org.apache.commons.lang3.exception;
    exports org.apache.commons.lang3.math;
    exports org.apache.commons.lang3.mutable;
    exports org.apache.commons.lang3.reflect;
    exports org.apache.commons.lang3.text;
    exports org.apache.commons.lang3.text.translate;
    exports org.apache.commons.lang3.time;
    exports org.apache.commons.lang3.tuple;
}

Upvotes: 3

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