Summary
-------

Strongly encapsulate all internal elements of the JDK by default, except
for [critical internal APIs][crit] such as `sun.misc.Unsafe`.  Allow end
users to choose the relaxed strong encapsulation that has been the
default since JDK 9.

[crit]: https://openjdk.java.net/jeps/260#Description


Goals
-----

  - Continue to improve the security and maintainability of the JDK,
    which is one of the primary goals of [Project Jigsaw][jig].

  - Encourage developers to migrate from using internal elements to using
    standard APIs, so that both they and their users can upgrade without
    fuss to future Java releases.


Non-Goals
---------

  - It is not a goal to remove, encapsulate, or modify any [critical
    internal APIs][crit] of the JDK for which standard replacements do
    not yet exist. **This means that `sun.misc.Unsafe` will remain
    available**.

  - It is not a goal to define new standard APIs to replace internal
    elements for which standard replacements do not yet exist, though
    such APIs could be suggested in response to this JEP.


Motivation
----------

Over the years the developers of various libraries, frameworks, tools,
and applications have used internal elements of the JDK in ways that
compromise both security and maintainability.  In particular:

  - Some non-`public` classes, methods, and fields of `java.*` packages
    define privileged operations such as [the ability to define a new
    class in a specific class loader][cldc], while others convey
    sensitive data such as [cryptographic keys][keystore].  These
    elements are internal to the JDK, despite being in `java.*` packages.
    The use of these internal elements by external code, via reflection,
    puts the security of the platform at risk.

  - All classes, methods, and fields of `sun.*` packages are internal
    APIs of the JDK.  Some classes, methods, and fields of `com.sun.*`,
    `jdk.*`, and `org.*` packages are also internal APIs.  These APIs
    were never standard, never supported, and [never intended for
    external use][sun-faq].  The use of these internal elements by
    external code is an ongoing maintenance burden.  Time and effort
    spent preserving these APIs, so as not to break existing code, could
    be better spent moving the platform forward.

In Java 9, we improved both the security and the maintainability of
the JDK by leveraging modules to [limit access to its internal
elements][jep260].  Modules provide _strong encapsulation_, which means
that

  - Code outside of a module can only access the `public` and `protected`
    elements of the packages exported by that module, and

  - `protected` elements can, further, only be accessed from subclasses
    of the classes that define them.

Strong encapsulation applies at both compile time and run time, including
when compiled code attempts to access elements via reflection at run
time.  The non-`public` elements of exported packages, and all elements
of unexported packages, are said to be _strongly encapsulated_.

In JDK 9 and later releases we strongly encapsulated all new internal
elements, thereby limiting access to them.  As an aid to migration,
however, we deliberately chose not to strongly encapsulate, at run time,
the content of packages that existed in JDK 8.  Library and
application code on the class path could thus continue to use reflection
to access the non-`public` elements of `java.*` packages, and all
elements of `sun.*` and other internal packages, for packages that
existed in JDK 8.  This arrangement is called [_relaxed strong
encapsulation_][relax].

We released JDK 9 in September 2017.  Most of the commonly-used
internal elements of the JDK now have [standard replacements][replace].
Developers have had over three years in which to migrate away from
internal elements of the JDK to standard APIs such as
`java.lang.invoke.MethodHandles.Lookup::defineClass`, `java.util.Base64`,
and `java.lang.ref.Cleaner`.  Many library, framework, and tool
maintainers have completed that migration and released updated versions
of their components.  We are now ready to take the next step toward the
strong encapsulation of all internal elements of the JDK — except for
critical internal APIs such as `sun.misc.Unsafe` — as originally planned
in Project Jigsaw.

[cldc]: https://docs.oracle.com/en/java/javase/15/docs/api/java.base/java/lang/ClassLoader.html#defineClass(java.lang.String,byte%5B%5D,int,int)
[keystore]: https://youtu.be/Vxfd3ehdAZc?t=1248
[jep260]: https://openjdk.java.net/jeps/260
[sun-faq]: http://web.archive.org/web/19980215011039/http://java.sun.com/products/jdk/faq/faq-sun-packages.html
[relax]: https://openjdk.java.net/jeps/261#Relaxed-strong-encapsulation
[se9-relax]: https://cr.openjdk.java.net/~iris/se/9/java-se-9-fr-spec-01/#Relaxing-strong-encapsulation
[jep261]: https://openjdk.java.net/jeps/261
[jig]: https://openjdk.java.net/projects/jigsaw
[replace]: https://wiki.openjdk.java.net/display/JDK8/Java+Dependency+Analysis+Tool#JavaDependencyAnalysisTool-ReplaceusesoftheJDK'sinternalAPIs
[jls-open]: https://docs.oracle.com/javase/specs/jls/se15/html/jls-7.html#jls-7.7.2


Description
-----------

Relaxed strong encapsulation is controlled by the launcher option
`--illegal-access`.  This option, introduced by [JEP 261][relax], is
provocatively named in order to discourage its use.  It presently works
as follows:

  - `--illegal-access=permit` arranges for every package that existed in
    JDK 8 to be [open][jls-open] to code in unnamed modules.  Code
    on the class path can thus continue to use reflection to access the
    non-public elements of `java.*` packages, and all elements of `sun.*`
    and other internal packages, for packages that existed in JDK 8.
    The first reflective-access operation to any such element causes a
    warning to be issued, but no warnings are issued after that point.

    This mode has been the default since JDK 9.

  - `--illegal-access=warn` is identical to `permit` except that a
    warning message is issued for every illegal reflective-access
    operation.

  - `--illegal-access=debug` is identical to `warn` except that both a
    warning message and a stack trace are issued for every illegal
    reflective-access operation.

  - `--illegal-access=deny` disables all illegal-access operations except
    for those enabled by other command-line options, _e.g._,
    `--add-opens`.

As the next step toward strongly encapsulating all internal elements of
the JDK, we propose to change the default mode of the `--illegal-access`
option from `permit` to `deny`.  With this change, packages that existed
in JDK 8 and do not contain [critical internal APIs][crit] will no
longer be open by default; a complete list is available
[here][jdk8-pkgs].  **The `sun.misc` package will still be exported by
the `jdk.unsupported` module, and will still be accessible via
reflection.**

We will also revise the related text in the [Java Platform
Specification][se9-relax] to disallow the opening of any package by
default in any Java Platform Implementation, unless that package is
explicitly declared to be `open` in the declaration of its containing
module.

The `permit`, `warn`, and `debug` modes of the `--illegal-access` option
will continue to work.  These modes allow end users to choose relaxed
strong encapsulation if they wish.

We expect a future JEP to remove the `--illegal-access` option entirely.
At that point it will not be possible to open all of the JDK 8
packages via a single command-line option.  It will still be possible to
use the [`--add-opens`][add-opens] command-line option, or the
[`Add-Opens`][modjar] JAR-file attribute, to open specific packages.

To prepare for the eventual removal of the `--illegal-access` option we
will deprecate it for removal as part of this JEP.  As a consequence,
specifying that option to the `java` launcher will cause a deprecation
warning to be issued.

[jdk9]: https://openjdk.java.net/projects/jdk9/
[jdk8-pkgs]: https://cr.openjdk.java.net/~mr/jigsaw/jdk8-packages-denied-by-default
[add-opens]: https://openjdk.java.net/jeps/261#Breaking-encapsulation
[modjar]: https://openjdk.java.net/jeps/261#Packaging:-Modular-JAR-files


Risks and Assumptions
---------------------

The primary risk of this proposal is that existing Java code will fail to
run.  The kinds of code that will fail include, but are not limited, to:

  - Frameworks that use the `protected` `defineClass` methods of
    `java.lang.ClassLoader` in order to define new classes in existing
    class loaders.  Such frameworks should instead use
    `java.lang.invoke.MethodHandles.Lookup::defineClass`, which has been
    available since JDK 9.

  - Code that uses the `sun.util.calendar.ZoneInfo` class to manipulate
    time-zone information.  Such code should instead use the `java.time`
    API, available since JDK 8.

  - Code that uses the `com.sun.rowset` package to process SQL row sets.
    Such code should instead use the `javax.sql.rowset` package,
    available since JDK 7.

  - Tools that use the `com.sun.tools.javac.*` packages to process source
    code.  Such tools should instead use the `javax.tools`,
    `javax.lang.model`, and `com.sun.source.*` APIs, available since
    JDK 6.

  - Code that uses the `sun.security.tools.keytool.CertAndKeyGen` class
    to generate self-signed certificates.  There is not yet a standard
    API for this functionality (though a [request has been
    submitted][jbs-certgen]); in the mean time, developers can use
    existing third-party libraries that include this functionality.

[jbs-certgen]: https://bugs.openjdk.java.net/browse/JDK-8058778

  - Code that uses the JDK’s internal copy of the Xerces XML processor.
    Such code should instead use a standalone copy of the Xerces library,
    [available from Maven Central][mvn-xerces].

[mvn-xerces]: https://search.maven.org/artifact/xerces/xercesImpl

  - Code that uses the JDK's internal version of the ASM bytecode
    library.  Such code should instead use a standalone copy of the ASM
    library, [available from Maven Central][mvn-asm].

[mvn-asm]: https://search.maven.org/artifact/org.ow2.asm/asm

We encourage all developers to:

  - Use the `jdeps` tool to identify code that depends upon internal
    elements of the JDK.

      - When [standard replacements][replace] are available, switch to
        using those.

      - Otherwise, we welcome strong cases for new standard APIs on the
        [Project Jigsaw mailing list][jigsaw-dev].  Please understand,
        however, that we are unlikely to define new standard APIs for
        internal elements that are not broadly used.

  - Use an existing release, such as JDK 11, to test existing code
    with `--illegal-access=warn` to identify any internal elements
    accessed via reflection, then use `--illegal-access=debug` to
    pinpoint the errant code, and then finally test with
    `--illegal-access=deny`.

[jigsaw-dev]: https://mail.openjdk.java.net/mailman/listinfo/jigsaw-dev


### Secondary risks

  - An existing application may fail to run not because the application
    itself makes use of internal APIs, but because the application uses
    libraries or frameworks which do so.  If you maintain such an
    application then we recommend that you update to the latest versions
    of the components upon which your application depends.  If those
    components have not yet been updated to remove dependencies upon
    internal elements then we suggest that you urge their maintainers to
    do so, or perhaps consider doing that work yourself and submitting a
    patch.

  - The maintainers of some libraries, frameworks, and tools have been
    telling application developers that illegal reflective-access
    warnings can safely be ignored when using JDK 9 and later.  This
    causes tension with application developers who always use the very
    latest JDK release and realize that the components upon which they
    depend will break as soon as the JDK’s internal elements are strongly
    encapsulated by default.  For these application developers,
    downgrading to JDK 8 or not moving to the latest release is not
    a viable approach.


### Examples of the impact of this change

  - Code successfully compiled with earlier releases that directly
    accesses internal APIs of the JDK will no longer work by default.
    For example,

    ```
    System.out.println(sun.security.util.SecurityConstants.ALL_PERMISSION);
    ```

    will fail with an exception of the form

    ```
    Exception in thread "main" java.lang.IllegalAccessError: class Test
      (in unnamed module @0x5e481248) cannot access class
      sun.security.util.SecurityConstants (in module java.base) because
      module java.base does not export sun.security.util to unnamed
      module @0x5e481248
    ```

  - Code that uses reflection to access `private` fields of exported
    `java.*` APIs will no longer work by default.  For example,

    ```
    var ks = java.security.KeyStore.getInstance("jceks");
    var f = ks.getClass().getDeclaredField("keyStoreSpi");
    f.setAccessible(true);
    ```

    will fail with an exception of the form

    ```
    Exception in thread "main" java.lang.reflect.InaccessibleObjectException:
      Unable to make field private java.security.KeyStoreSpi
      java.security.KeyStore.keyStoreSpi accessible: module java.base does
      not "opens java.security" to unnamed module @6e2c634b
    ```

  - Code that uses reflection to invoke `protected` methods of exported
    `java.*` APIs will no longer work by default.  For example,

    ```
    var dc = ClassLoader.class.getDeclaredMethod("defineClass",
                                                 String.class,
                                                 byte[].class,
                                                 int.class,
                                                 int.class);
    dc.setAccessible(true);
    ```

    will fail with an exception of the form

    ```
    Exception in thread "main" java.lang.reflect.InaccessibleObjectException:
      Unable to make protected final java.lang.Class
      java.lang.ClassLoader.defineClass(java.lang.String,byte[],int,int)
      throws java.lang.ClassFormatError accessible: module java.base does
      not "opens java.lang" to unnamed module @5e481248
    ```