Strongly encapsulate all internal elements of the JDK, except for
critical internal APIs such as sun.misc.Unsafe. It will no
longer be possible to relax the strong encapsulation of internal elements
via a single command-line option, as was possible in JDK 9 through
JDK 16.
This JEP is the successor to JEP 396, which transitioned the JDK from a default of relaxed strong encapsulation to a default of strong encapsulation, while allowing users to return to the relaxed posture if they wished. The Goals, Non-Goals, Motivation, and Risks and Assumptions section of this JEP are essentially identical to that of JEP 396 but are reproduced here for the reader’s convenience.
Continue to improve the security and maintainability of the JDK, which is one of the primary goals of Project Jigsaw.
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.
It is not a goal to remove, encapsulate, or modify any critical
internal APIs 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.
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, while others convey
sensitive data such as cryptographic keys. 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. Most 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. 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. 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 internal elements that had 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, and was the default behavior in JDK 9.
We released JDK 9 back in September 2017. Most of the commonly-used
internal elements of the JDK now have standard replacements.
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. The need for relaxed strong encapsulation is weaker
now than it was in 2017, and it weakens further every year.
In JDK 16, released in March 2021, we took the next step toward
strongly encapsulating all internal elements of the JDK. JEP
396 made strong encapsulation the default behavior except for
critical internal APIs such as sun.misc.Unsafe, which remained
available. In JDK 16 it was still possible for end users to choose
relaxed strong encapsulation in order to gain access to internal elements
that existed in JDK 8.
We are now ready to take one more step in this journey by removing the
ability to choose relaxed strong encapsulation. This means that all
internal elements of the JDK will be strongly encapsulated except for
critical internal APIs such as sun.misc.Unsafe.
Relaxed strong encapsulation is controlled by the launcher option
--illegal-access. This option, introduced by JEP 261,
was provocatively named in order to discourage its use. In JDK 16
and earlier releases, it works as follows:
--illegal-access=permit arranges for every package that existed in
JDK 8 to be 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 was the default from JDK 9 through JDK 15.
--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.
This mode was the default in JDK 16.
As the next step toward strongly encapsulating all internal elements of
the JDK, we propose to make the --illegal-access option obsolete. Any
use of this option, whether with permit, warn, debug, or deny,
will have no effect other than to issue a warning message. We expect to
remove the --illegal-access option entirely in a future release.
With this change, it will no longer be possible for end users to use the
--illegal-access option to enable access to internal elements of the
JDK. (A list of the packages affected is available here.)
The sun.misc and sun.reflect packages will still be exported by the
jdk.unsupported module, and will still be open so that code can access
their non-public elements via reflection. No other JDK packages will be
open in this way.
It will still be possible to use the --add-opens
command-line option, or the Add-Opens JAR-file manifest
attribute, to open specific packages.
com.sun APIsMost com.sun.* packages in the JDK are for internal use, but a few are
supported for external use. These supported packages were exported in
JDK 9 and will continue to be exported, so you can continue to
program against their public APIs. They will, however, no longer be
open. Examples include
jdk.compiler module,jdk.httpserver module,jdk.sctp module, andcom.sun.nio.file
package of the jdk.unsupported module.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); in the mean time, developers can use
existing third-party libraries that include this functionality.
We encourage all developers to:
Use the jdeps tool to identify code that depends upon internal
elements of the JDK.
When standard replacements are available, switch to using those.
Otherwise, we welcome strong cases for new standard APIs on the Project Jigsaw mailing list. 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.
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. For these application developers, downgrading to JDK 8 or not moving to the latest release is not a viable approach.
Code successfully compiled with earlier releases that directly accesses internal APIs of the JDK will no longer work. 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 @0x5e481248Code that uses reflection to access private fields of exported
java.* APIs will no longer work. 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 @6e2c634bCode that uses reflection to invoke protected methods of exported
java.* APIs will no longer work. 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