I am trying to understand how javac deals with cyclic imports. After few attempts, I got the below code to be compiled:
package com.test.p1;
import com.test.p2.ClassP2;
public class ClassP1 {
public static void main(String[] args) {
System.out.println("ClassP1 loaded");
ClassP2.testP2();
}
}
And
package com.test.p2;
import com.test.p1.ClassP1;
public class ClassP2 {
public static void testP2() {
System.out.println("ClassP2 loaded");
}
}
When I compiled ClassP2.java first, I was expecting it to fail as ClassP1.java was not yet compiled and no class file was generated. Yet, it compiled ok, and generated ClassP2.class and ClassP1.class. Running ClassP1 worked fine as well.
Now my question is how did javac generate class file for ClassP1 when I only compiled ClassP2. And why does this not happen when cyclic reference is not there - i.e. when ClassP2 imports ClassP1, but ClassP1 does not import ClassP2 ?
I am still new to Java and trying to understand how compilation works.
There's nothing cyclic going on here. It sounds like you, along with a lot of other people, confuse import with class loading.
All the import statement does is allow you to use the short name to refer to a Java class in your code:
import java.sql.ResultSet;
public V find() {
ResultSet rs = null; // You can type ResultSet instead of java.sql.ResultSet
}
The import statement won't help you if the short name is ambiguous (e.g. java.util.Date and java.sql.Date in the same class).
The byte code for java.sql.ResultSet is not loaded into the JVM until your code needs it at runtime.
It is not a result of cyclic imports, it is a result of importing ClassP1 in ClassP2.java and javac being able to find the source for ClassP1 based on the arguments you passed to javac and the location from which you executed the command.
See the -sourcepath option in the docs for javac:
-sourcepath sourcepath
Specify the source code path to search for class or interface definitions. As with the user class path, source path entries are separated by semicolons (;) and can be directories, JAR archives, or ZIP archives. If packages are used, the local path name within the directory or archive must reflect the package name.
Note: Classes found through the class path may be subject to automatic recompilation if their sources are also found. See Searching For Types.
Related
I've the following two source files
File World.java
package planets;
public class World {
public static void main(String[] args) {
Mars.land();
}
}
File Moon.java
package planets;
public class Moon {
public static void land() {
System.out.println("Hello Moon");
}
}
class Mars {
public static void land() {
System.out.println("Hello Mars");
}
}
As we can see, the Moon.java contains two classes: the public Moon class and the nonpublic Mars class.
The files are located inside planets directory, below is showed the directory tree
+current-dir:
+----+planets:
+----+World.java
+----+Moon.java
Now, if I try to compile from Windows command prompt (I'm inside current-dir folder) typing
javac planets\World.java
I receive this error message:
planets\World.java:5: error: cannot find symbol
Mars.land();
^
symbol: variable Mars
location: class World
1 error
It's very strange, because I know that the compiler searches for nonpublic classes inside all the source files of the current package.
Also Cay Horstmann's Core Java Vol 1, 10th ed. at pp. 192-193 says that:
[...]you can import nonpublic classes from the current package. These
classes may be defined in source files with different names. If you
import a class from the current package, the compiler searches all
source files of the current package to see which one defines the
class.
In addition I tried to write these files using Eclipse Oxygen and it compile without problems. But I know that Eclipse use a different compiler.
Why does javac compiler fail?
EDIT: I have not set CLASSPATH variable. So by default compiler looks inside current directory.
you need to type the following commands in order (inside your 'current-dir')
javac planets\Moon.java
javac -cp . planets\World.java
java -cp . planets.World
Example:
I have a class called ProgA
package test;
public class ProgA
{
public static void main(String[] args)
{
ProgB pb = new ProgB();
pb.callMe();
}
}
Now I have the ProgB like below:
package test2;
public class ProgB
{
public void callMe()
{
System.out.println("inside callme");
}
}
After compiling ProgB.java its class file is generated in the test2 package. Now when I try to compile ProgA.java using this command:
javac -cp C:\Users\MyName\Desktop\test2 ProgA.java
I get the error that it cannot find ProgB.
My question is why cant java look inside the class path to find ProgB.class file and compile my ProgA.java successfully? The code works fine when I specify the fully qualified class name of ProgB inside ProgA.java code and run with the classpath set to -classpath C:\Users\MyName\Desktop. Why to have the fully quilified name when I am already specifying the full class path to find ProgB. I am not clear with that concept of classpath and fully qualified class name. Please explain me. Thank you
First you would need to import the class. This is why it asks you to use a fully qualified class name. You cannot use a class that is not in the same package without importing it (or using the fully qualified class name).
import test2.ProgB;
Then while compiling, you should provide the class path till the root location, the compiler will look for the class using the package name as the path.
Your compile command should be.
javac -cp C:\Users\MyName\Desktop ProgA.java
In order to use a class from another package, you need to either use the fully qualified class name, or have an import statement. This is a .java source code requirement. It can't be fixed simply by fiddling with the compiler's classpath.
Without an import statement, unqualified names are assumed to belong to the current source file's package. If you're in a package test file, the identifier ProgB will match test.ProgB but not test2.ProgB. The compiler won't search other packages unless you tell it to.
We tried to import a test rascal module and a module from the standard library using JavaToRascal.
The test module is stored in C:\Users\Klemens\workspace\RascalInterop\src\MyTest.rsc and contains:
module MyTest
Te java code containing the JavaToRascal invocation is as follows:
import java.io.PrintWriter;
import java.net.URISyntaxException;
import org.rascalmpl.interpreter.JavaToRascal;
import org.rascalmpl.interpreter.load.IRascalSearchPathContributor;
import org.rascalmpl.interpreter.load.StandardLibraryContributor;
import org.rascalmpl.interpreter.load.URIContributor;
import org.rascalmpl.uri.URIUtil;
public class RascalInterop {
public static void main(String[] args) throws URISyntaxException {
JavaToRascal j2r = new JavaToRascal(new PrintWriter(System.out), new PrintWriter(System.err));
IRascalSearchPathContributor modulePath = new URIContributor(URIUtil.createFileLocation("C:\\Users\\Klemens\\workspace\\RascalInterop\\src\\MyTest.rsc"));
j2r.getEvaluator().addRascalSearchPathContributor(modulePath);
try {
j2r.eval("import MyTest;").toString(); // Could not import module MyTest: can not find in search path
} catch (Exception e) {
System.out.println(e.getMessage());
}
try {
j2r.getEvaluator().addRascalSearchPathContributor(StandardLibraryContributor.getInstance());
j2r.eval("import IO;").toString(); // null pointer exception
} catch (Exception e) {
System.out.println(e.getClass());
}
}
}
The print in the first try block that tries to import our MyTest.rsc module results in:
Could not import module MyTest: can not find in search path
?[Advice](http://tutor.rascal-mpl.org/Errors/Static/ModuleImport/ModuleImport.html)
The second import attempting to import the IO module from the standard library results in:
class java.lang.NullPointerException
Any ideas how to use properly set the search path from a Java program?
We tried to use j2r.getEvaluator().addRascalSearchPathContributor in various ways but did not succeed in loading a MyTest.rsc module from the given directory.
Despite that these API will change in the near future (due to the compilation process and related changes), here's an answer. Two answers actually, one for Rascal files and one for Java code that it needs
For Rascal:
j2r.getEvaluator().addRascalSearchPathContributor
What you used is the correct way of doing things. So if it did not work, please provide more code so we can diagnose what goes wrong. So where is your module? Is it in a jar file or a binary folder? If its in a jar, you need some additional wiring I'm glad to explain.
The Rascal search path is distinguished from the Classpath for Java classes which are used by Rascal. So you have different API for that. We use classloaders to find Java files (such that it also works for situations like OSGI bundles in Eclipse):
Evaluator x = ctx.getEvaluator();
x.addClassLoader(getClass().getClassLoader());
This will make sure that the class loader used to load the current class is also used to load the class linked mentioned in the Rascal file. Of course you can also provide other class loaders. Note that if the libraries you depend on are loaded via OSGI, make sure you get a classloader from a class that is in a bundle that has access to these classes. The simple case is when everything is in the same jar file, then any classloader will do.
I think you should change the path to refer to the src directory instead of the source file:
new URIContributor(URIUtil.createFileLocation("C:\\Users\\Klemens\\workspace\\RascalInterop\\src"));
Also: probably you should use forward slashes without C:\, so /Users/.../src
AFAIK The null pointer exception is expected, evaluating import returns null, and you try to call toString().
When preparing for the SCJP exam, we were going through the following code:
package certificaton;
public class OtherClass
{
public void testIt()
{
System.out.println("otherclass");
}
}
And this:
package somethingElse;
import certification.OtherClass;
public class AccessClass
{
public static void main( String args[])
{
OtherClass o= new OtherClass();
o.testIt();
}
}
I placed both the above files in the following directory: C:\scjp\temp8 ; and the strange thing is that, the .java files are compiling and results in two .class files being created in the same directory. The thing I want to ask, is that, the difference between packages and directory. Isn't it true that the class files could be created in a directory other than the one stated in the package declaration? And the package declaration is something 'virtual', and disregards the windows directory structure. In addition, isn't it also true that, by executing the following command:
javac -d . OtherClass.java
The directories are created conforming to the package declaration, which isn't always mandatory?
The directories are created conforming
to the package declaration, which
isn't always mandatory?
No, the package and directory structures must match. It's mandatory, not optional.
I am running into this problem when trying to recompile a single class inside a package.
Now this class uses global types and some of these global types reference it. So taking it out of the package really isn't an option.
So when I try to compile it with javac alone, I get invalid symbol errors and netbeans shows it is trying to compile things like classespackage.globaltype. Basically it is searching for the global classes inside of the package. Is there anyway to stop it from doing that?
Here is the code:
Global
public class Global {
example.Main main;
public Global(example.Main m) {
main = m;
}
}
example.Main
package example;
public class Main {
public static void main(String[] args) {
Global g = new Global(new Main()); // COMPILE ERROR
}
}
I get invalid symbol errors
You probably meant "Cannot find symbol" errors? This can be caused by anything. Imported class which is not in the compiletime classpath, methods which does not exist, variables which are out of the scope. You really need to post the compilation errors to get more detailed answers.
At least, this much sounds like that you didn't specify the dependencies (the imported classes) in the compiletime classpath using the -cp or -classpath argument.
Is there anyway to stop it from doing that?
By listening to those errors and taking actions accordingly.
Update as per the posted code example: the cause of the problem is that classes in the default package (i.e. classes without a package declaration) are invisible to classes inside a concrete package (i.e. classes with a package declaration). You need to put Global in a package. Then it's visible (importable) to classes inside a package.
If you reference the global types in a class package, you'll have to include them in your classpath. When you do javac, make sure you include the global types class in your classpath (with the -cp option.)