JEP 476: Module Import Declarations (Preview)
Author | Jim Laskey & Gavin Bierman |
Owner | Gavin Bierman |
Type | Feature |
Scope | SE |
Status | Closed / Delivered |
Release | 23 |
Component | specification / language |
Discussion | amber dash dev at openjdk dot org |
Effort | S |
Duration | S |
Relates to | JEP 477: Implicitly Declared Classes and Instance Main Methods (Third Preview) |
Reviewed by | Alex Buckley, Brian Goetz |
Endorsed by | Brian Goetz |
Created | 2023/08/28 16:57 |
Updated | 2024/09/03 13:42 |
Issue | 8315129 |
Summary
Enhance the Java programming language with the ability to succinctly import all of the packages exported by a module. This simplifies the reuse of modular libraries, but does not require the importing code to be in a module itself. This is a preview language feature.
Goals
-
Simplify the reuse of modular libraries by allowing entire modules to be imported at once.
-
Avoid the noise of multiple type-import-on-demand declarations (e.g.,
import com.foo.bar.*
) when using diverse parts of the API exported by a module. -
Allow beginners to more easily use third-party libraries and fundamental Java classes without having to learn where they are located in a package hierarchy.
-
Do not require developers who use the module import feature to modularize their own code.
Motivation
Classes and interfaces in the java.lang
package, such as Object
, String
,
and Comparable
, are essential to every Java program. For this reason, the Java
compiler automatically imports, on demand, all the classes and interfaces in the
java.lang
package, as if
import java.lang.*;
appears at the beginning of every source file.
As the Java Platform has evolved, classes and interfaces such as List
, Map
,
Stream
, and Path
have become almost as essential. However, none of these are
in java.lang
, so they are not automatically imported; rather, developers have
to keep the compiler happy by writing a plethora of import
declarations at the
beginning of every source file. For example, the following code converts an
array of strings into a map from capital letters to strings, but the imports
take almost as many lines as the code:
import java.util.Map; // or import java.util.*;
import java.util.function.Function; // or import java.util.function.*;
import java.util.stream.Collectors; // or import java.util.stream.*;
import java.util.stream.Stream; // (can be removed)
String[] fruits = new String[] { "apple", "berry", "citrus" };
Map<String, String> m =
Stream.of(fruits)
.collect(Collectors.toMap(s -> s.toUpperCase().substring(0,1),
Function.identity()));
Developers have diverse views as to whether to prefer single-type-import or type-import-on-demand declarations. Many prefer single-type imports in large, mature codebases where clarity is paramount. However, in early-stage situations where convenience trumps clarity, developers often prefer on-demand imports; for example,
-
When prototyping code and running it with the
java
launcher; -
When exploring a new API in JShell, such as Stream Gatherers or the Foreign Function & Memory API; or
-
When learning to program with new features that work in concert with new APIs, such as virtual threads and their executors.
Since Java 9, modules have allowed a set of packages to be grouped together for reuse under a single name. The exported packages of a module are intended to form a cohesive and coherent API, so it would be convenient if developers could import on-demand from the entire module, that is, from all of the packages exported by the module. It would be as if all the exported packages are imported in one go.
For example, importing the java.base
module on-demand would give immediate
access to List
, Map
, Stream
, and Path
, without having to manually import
java.util
on-demand, and java.util.stream
on-demand, and java.nio.file
on-demand.
The ability to import at the level of modules would be especially helpful when
APIs in one module have a close relationship with APIs in another module. This
is common in large multi-module libraries such as the JDK. For example, the
java.sql
module provides database access via its java.sql
and javax.sql
packages, but one
of its interfaces,
java.sql.SQLXML
, declares public
methods whose signatures use interfaces from the
javax.xml.transform
package in the java.xml
module.
Developers who call these methods in java.sql.SQLXML
typically import both the
java.sql
package and the javax.xml.transform
package. To facilitate this
extra import, the java.sql
module depends on the java.xml
module
transitively, so that a
program which depends on the java.sql
module depends automatically on the
java.xml
module. In this scenario, it would be convenient if importing the
java.sql
module on-demand would also automatically import the java.xml
module on-demand. Automatically importing on-demand from transitive dependencies
would be a further convenience when prototyping and exploring.
Description
A module import declaration has the form
import module M;
It imports, on demand, all of the public
top-level classes and interfaces in
-
The packages exported by the module
M
to the current module, and -
The packages exported by the modules that are read by the current module due to reading the module
M
.
The second clause allows a program to use the API of a module, which might refer to classes and interfaces from other modules, without having to import all of those other modules.
For example:
-
import module java.base
has the same effect as 54 on-demand package imports, one for each of the packages exported by thejava.base
module. It is as if the source file containsimport java.io.*
andimport java.util.*
and so on. -
import module java.sql
has the same effect asimport java.sql.*
andimport javax.sql.*
plus on-demand package imports for the indirect exports of thejava.sql
module.
This is a preview language feature, disabled by default
To try the examples below in JDK 23, you must enable preview features:
-
Compile the program with
javac --release 23 --enable-preview Main.java
and run it withjava --enable-preview Main
; or, -
When using the source code launcher, run the program with
java --enable-preview Main.java
; or, -
When using
jshell
, start it withjshell --enable-preview
.
Syntax and semantics
We extend the grammar of import declarations (JLS §7.5) to include import module
clauses:
ImportDeclaration:
SingleTypeImportDeclaration
TypeImportOnDemandDeclaration
SingleStaticImportDeclaration
StaticImportOnDemandDeclaration
ModuleImportDeclaration
ModuleImportDeclaration:
import module ModuleName;
import module
takes a module name, so it is not possible to import packages
from the unnamed module, i.e., from the class path. This aligns with requires
clauses in module declarations, i.e., module-info.java
files, which take
module names and cannot express a dependence on the unnamed module.
import module
can be used in any source file. The source file need
not be associated with an explicit module. For example, java.base
and
java.sql
are part of the standard Java runtime, and can be imported by
programs which are not themselves developed as modules. (For technical
background, see JEP 261.)
It is sometimes useful to import a module that does not export any packages,
because the module transitively requires other modules that do export
packages. For example, the
java.se
module does not export any packages, but it requires 19 other modules
transitively, so the effect of import module java.se
is to import the
packages which are exported by those modules, and so on, recursively —
specifically, the 123 packages listed as the indirect exports of the java.se
module.
Note that import module java.se
is only possible in a compilation unit of a named module that already requires java.se
. In a compilation unit of the unnamed module, such as one that implicitly declares a class, it is not possible to use import module java.se
.
Ambiguous imports
Since importing a module has the effect of importing multiple packages, it is possible to import classes with the same simple name from different packages. The simple name is ambiguous, so using it will cause a compile-time error.
For example, in this source file the simple name Element
is ambiguous:
import module java.desktop; // exports javax.swing.text,
// which has a public Element interface,
// and also exports javax.swing.text.html.parser,
// which has a public Element class
...
Element e = ... // Error - Ambiguous name!
...
As another example, in this source file the simple name List
is ambiguous:
import module java.base; // exports java.util, which has a public List interface
import module java.desktop; // exports java.awt, which a public List class
...
List l = ... // Error - Ambiguous name!
...
As a final example, in this source file the simple name Date
is ambiguous:
import module java.base; // exports java.util, which has a public Date class
import module java.sql; // exports java.sql, which has a public Date class
...
Date d = ... // Error - Ambiguous name!
...
Resolving ambiguities is straightforward: Use a single-type-import
declaration. For example, to resolve the ambiguous Date
of the previous
example:
import module java.base; // exports java.util, which has a public Date class
import module java.sql; // exports java.sql, which has a public Date class
import java.sql.Date; // resolve the ambiguity of the simple name Date!
...
Date d = ... // Ok! Date is resolved to java.sql.Date
...
A worked example
Here is an example of how import module
works. Suppose C.java
is a source
file associated with module M0
:
// C.java
package q;
import module M1; // What does this import?
class C { ... }
where module M0
has the following declaration:
module M0 { requires M1; }
The meaning of import module M1
depends on the exports of M1
and any
modules that M1
requires transitively.
module M1 {
exports p1;
exports p2 to M0;
exports p3 to M3;
requires transitive M4;
requires M5;
}
module M3 { ... }
module M4 { exports p10; }
module M5 { exports p11; }
The effect of import module M1
is to
-
Import the
public
top level classes and interfaces from packagep1
, sinceM1
exportsp1
to everyone; -
Import the
public
top level classes and interfaces from packagep2
, sinceM1
exportsp2
toM0
, the module with whichC.java
is associated; and -
Import the
public
top level classes and interfaces from packagep10
, sinceM1
requires transitivelyM4
, which exportsp10
.
Nothing from packages p3
or p11
is imported by C.java
.
Implicitly declared classes
This JEP is co-developed with JEP 477: Implicitly Declared Classes and
Instance main
Methods, which specifies that
all public
top level classes and interfaces in all packages exported by the
java.base
module are automatically imported on-demand in implicitly declared
classes. In other words, it is as if import module java.base
appears at the
beginning of every such class, versus import java.lang.*
at the beginning of
every ordinary class.
The JShell tool automatically imports ten
packages on-demand. The list of packages is ad-hoc. We therefore propose to
change JShell to automatically import module java.base
.
Alternatives
-
An alternative to
import module ...
is to automatically import more packages than justjava.lang
. This would bring more classes into scope, i.e., usable by their simple names, and delay the need for beginners to learn about imports of any kind. But, which additional packages should we import automatically?Every reader will have suggestions for which packages to auto-import from the omnipresent
java.base
module:java.io
andjava.util
would be near-universal suggestions;java.util.stream
andjava.util.function
would be common; andjava.math
,java.net
, andjava.time
would each have supporters. For the JShell tool, we managed to find tenjava.*
packages which are broadly useful when experimenting with one-off Java code, but it is difficult to see which subset ofjava.*
packages deserves to be permanently and automatically imported into every Java program. The list would, moreover, change change as the Java Platform evolves; e.g.,java.util.stream
andjava.util.function
were introduced only in Java 8. Developers would likely become reliant on IDEs to remind them of which automatic imports are in effect — an undesirable outcome. -
An important use case for this feature is to automatically import on-demand from the
java.base
module in implicitly declared classes. This could alternatively be achieved by automatically importing the 54 packages exported byjava.base
. However, when an implicit class is migrated to an ordinary explicit class, which is the expected lifecycle, the developer would either have to write 54 on-demand package imports, or else figure out which imports are necessary.
Risks and Assumptions
Using one or more module import declarations leads to a risk of name ambiguity due to different packages declaring members with the same simple name. This ambiguity is not detected until the ambiguous simple name is used in a program, when a compile-time error will occur. The ambiguity can be resolved by adding a single-type-import declaration, but managing and resolving such name ambiguities could be burdensome and lead to code that is brittle and difficult to read and maintain.