ALGORITHM _> HOW JAVA COMPILER SEARCHES FOR CLASS FILE

Hola folks, Welcome again !!!!!,

I know it’s been so long but i am back again with very mind blowing and interesting blog on java compiler. This blog will give you an idea about java compiler workflow.

We will be seeing how java compiler searches for class definition…

First Create Two Java Files for testing [ Main.java, Sample.java]

SOURCE CODE

GitHub-link = https://github.com/tamimdalwai/JAVA-BLOG.git

Compile the Main.java and Sample.java File as a result two packages are generated “PackageMain” and “PackageSample”.

CASE 1 : Searches class Sample{ } in current method as LOCAL INNER CLASS

Now let’s execute the Main class

COMMAND -> java PackageMain.Main

We can see , compiler first checked inside local main method for Sample class.

CASE 2: Searches class Sample{ } in Main.class as CLASS LEVEL INNER CLASS

Now let’s comment the Method Level Inner Class Sample and now compiler will search one step above i.e. in Class Level Inner Class in the same Main.class file.

Searches for class in class level inner class

Before working further delete the generated packages and again compile the “Main.java” file again with the help of javac command.

Run ->> java PackageMain.Main

CLASS LEVEL INNER CLASS IS EXECUTED

CASE 3 : Searches class Sample { } in current java file (Main.java) as OUTER CLASS.

Now again do the same comment the Class Level Inner Class.

SEARCHES FOR CLASS FILE IN THE OUTER CLASS

Before working further delete the generated packages and again compile the “Main.java” file again with the help of javac command.

Run ->> java PackageMain.Main

OUTER CLASS IS EXECUTED

CASE 4 : Searches for class Sample{ } in CURRENT PACKAGE

Before working further delete the generated packages. Now manually create package “PackageMain” and create one Sample.java file inside that folder. Code for Sample.java

Package PackageMain;
public class Sample{
  void method(){
    System.out.println("Method executed from Sample.java file and PackageMain package i.e. current package");
  }
}

CREATE DIRECTORY AND CREATE AND COMPILE SAMPLE.JAVA AND MAIN.JAVA

Run ->> java PackageMain.Main

CLASS EXECUTED FROM CURRENT PACKAGE

CASE 5 : Searches for class Sample{ } in IMPORTED PACKAGE

Delete the PackageMain folder and now if we try to compile Main.java file we get below error (cannot find symbol)

So basically compiler is unable to find class, we have to explicitly set the class path as below:

SET CLASSPATH=.;PackageSample/*.class

Classpath is used by compiler and jvm to find the class files, here “.” means current working directory and “PackageSample\*.class” means all class files from PackageSample folder.After we have to import that Sample.class file with the help of import keyword.

import class files from PackageSample folder

Now if we try to compile the Main.java it successfully creates PackageMain and also we can see that there is no Sample.class available in the current package directory

Run ->> java PackageMain.Main

Searched class file from classpath

CONCLUSION: JAVAC ALGORITHM FOR SEARCHING CLASS FILE

1) Searches class Sample{ } in current method as local inner class,if not available
2) Searches class Sample{ } in class as class level inner class,if not available
3) Searches for class Sample { } in current java file as outer class,if not available
4) Searches for class Sample{ } in Current Package folder,if not available
5) Searches for class Sample{ } in imported package,if not available

6) Throws error

😃 Thanks for reading !!!

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