Java open and opens keyword with example
We have come a long way to exploring the Java Platform Module System.
Java 9 using exports and requires
Export to specific modules only
Transitive Dependency in Java 9
In this post we will look at open and opens keyword.
To make a long story short, we use open and opens keyword on module-info.java file, if we want to provide reflection API access to module packages.
We will be working with following directory structure. (All files are empty right now)
└── src
├── com.exported
│ ├── com
│ │ └── exported
│ │ └── Exported.java
│ └── module-info.java
├── com.user
│ ├── com
│ │ └── user
│ │ └── User.java
│ └── module-info.java
└── com.usertwo
├── com
│ └── usertwo
│ └── UserTwo.java
└── module-info.java
Let’s look at our first module com.exported. module-info.java file is simple
module com.exported {
exports com.exported;
}
This com.exported/module-info.java simply defines module and exportes some packages. This is nothing we haven’t seen yet.
Here’s a content of com.exported.Exorted.java
package com.exported;
public class Exported {
public String callMe() {
return "callMe";
}
}
For now this class is simple, with just one public method.
Notice: we haven’t used open / opens yet.
In module com.user module-info.java file we have fairly simple code as well.
module com.user {
requires com.exported;
}
If we look at com.user.User.java we have code that uses reflection.
package com.user;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
public class User {
public static void main(String[] args) throws Exception {
Class<?> exportedClass = Class.forName("com.exported.Exported");
Constructor<?> con = exportedClass.getDeclaredConstructor();
con.setAccessible(true);
Object exportedObj = con.newInstance();
Method m;
m = exportedObj.getClass().getMethod("callMe");
m.setAccessible(true);
System.out.println("Public : " + m.invoke(exportedObj));
}
}
If we compile and run the program we get following result
javac -d mods --module-source-path src $(find src -name "*.java")
java --module-path mods -m com.user/com.user.User
--- OUTPUT ---
Public : callMe
Wait!!! #
Earlier in the post I said that I had to use open / opens keyword if I wanted to give reflection API access to modules code. Why am I able to use reflection now then?. Of course because this is publicly available exported package.
Let’s add some code to twist this example. On com.exported.Exported.java add one private method.
package com.exported;
public class Exported {
public String callMe() {
return "callMe";
}
private String callMePrivate() {
return "callMePrivate";
}
}
Now on com.user.User.java let’s add reflection code to access that method.
package com.user;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
public class User {
public static void main(String[] args) throws Exception {
Class<?> exportedClass = Class.forName("com.exported.Exported");
Constructor<?> con = exportedClass.getDeclaredConstructor();
con.setAccessible(true);
Object exportedObj = con.newInstance();
Method m;
m = exportedObj.getClass().getMethod("callMe");
m.setAccessible(true);
System.out.println("Public : " + m.invoke(exportedObj));
m = exportedObj.getClass().getDeclaredMethod("callMePrivate");
m.setAccessible(true);
System.out.println("Private : " + m.invoke(exportedObj));
}
}
Compile and run it and we will get following output
javac -d mods --module-path ./jars/ --module-source-path src $(find src -name "*.java")
java --module-path ./jars/:mods -m com.user/com.user.User
--- OUTPUT ---
Public : callMe
Exception in thread "main" java.lang.reflect.InaccessibleObjectException: Unable to make private java.lang.String com.exported.Exported.callMePrivate() accessible: module com.exported does not "opens com.exported" to module com.user
at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:337)
at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:281)
at java.base/java.lang.reflect.Method.checkCanSetAccessible(Method.java:198)
at java.base/java.lang.reflect.Method.setAccessible(Method.java:192)
at com.user/com.user.User.main(User.java:22)
Notice we were able to access the public method with help of reflection but not private method. And the error clearly states that we are not adding statement opens com.exported. So let’s do that on com.exported/module-info.java.
module com.exported {
exports com.exported;
opens com.exported;
}
Now if we compile and run the code we will get the following result
javac -d mods --module-path ./jars/ --module-source-path src $(find src -name "*.java")
java --module-path ./jars/:mods -m com.user/com.user.User
--- OUTPUT ---
Public : callMe
Private : callMePrivate
We can take this even further. Because we are using reflection to access the public method as well we can remote exports com.exported statement from com.exported/module-info.java.
We used opens keyword and there more to it, but before going further on opens I want to talk about open keyword. So we use open keyword when we want to make all of the packages on the module to be accessed by reflection API. Because this is for all of the packages we use open on module decleration level.
open module com.exported {
}
If you compile and run the command now it will run fine. But for the moment let’s step back and foucus more on opens keyword. You know now that opens is used if we want selected package to be accessed by reflection API. But what if you want to give open access to only certain packages. Like exports .. to, we have opens .. to which enables use to give a fine grain access control mechanism even for reflection API.
Let’s see this in action shall we.
For this we will update our com.exported/module-info.java so that it opens its packages only to com.user module.
module com.exported {
opens com.exported to com.user;
}
If you run the program now of course it runs right. By the if you are thinking why I removed open keyword we can’t combine those.
So com.user module code remains same.
On our third module com.usertwo let’s add some code (Basically same as com.user)
Here’s com.usertwo/module-info.java
module com.usertwo {
requires com.exported;
}
package com.usertwo;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
// This class cannot use reflection API for com.exporter.Exported
public class UserTwo {
public static void main(String[] args) throws Exception {
Class<?> exportedClass = Class.forName("com.exported.Exported");
Constructor<?> con = exportedClass.getDeclaredConstructor();
con.setAccessible(true);
Object exportedObj = con.newInstance();
Method m;
m = exportedObj.getClass().getMethod("callMe");
m.setAccessible(true);
System.out.println("Public : " + m.invoke(exportedObj));
m = exportedObj.getClass().getDeclaredMethod("callMePrivate");
m.setAccessible(true);
System.out.println("Private : " + m.invoke(exportedObj));
}
}
If we run com.user/com/user/User.java as main class we will run our program successfully. But let’s try to run our program with main class com.usertwo/com/usertwo/UserTwo.java.
javac -d mods --module-path ./jars/ --module-source-path src $(find src -name "*.java")
java --module-path ./jars/:mods -m com.usertwo/com.usertwo.UserTwo
--- OUTPUT ---
Exception in thread "main" java.lang.reflect.InaccessibleObjectException: Unable to make public com.exported.Exported() accessible: module com.exported does not "exports com.exported" to module com.usertwo
at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:337)
at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:281)
at java.base/java.lang.reflect.Constructor.checkCanSetAccessible(Constructor.java:192)
at java.base/java.lang.reflect.Constructor.setAccessible(Constructor.java:185)
at com.usertwo/com.usertwo.UserTwo.main(UserTwo.java:12)
We will get this kind of error, it’s because we have delecated opens to com.user only. And for other package this is still not accesssable.
By the way, did you notice we were using requires com.exported; on both module. Even thought we are using reflection API to access the code. We need to declare that we are using code from that module or else we will get a error.
avac -d mods --module-path ./jars/ --module-source-path src $(find src -name "*.java")
java --module-path ./jars/:mods -m com.user/com.user.User
--- OUTPUT ---
Exception in thread "main" java.lang.ClassNotFoundException: com.exported.Exported
at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:582)
at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:185)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:496)
at java.base/java.lang.Class.forName0(Native Method)
at java.base/java.lang.Class.forName(Class.java:292)
at com.user/com.user.User.main(User.java:10)
So this is it for open and opens keyword. You can find the Github code here