Java - ClassLoader

向右看齐 2022-06-02 05:53 187阅读 0赞

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Java ClassLoader is one of the crucial but rarely used components of Java in Project Development. Personally I have never extended ClassLoader in any of my projects but the idea of having my own ClassLoader that can customize the Java Class Loading thrills me.

This article will provide an overview of Java ClassLoader and then move forward to create a custom ClassLoader in Java.

# What is Java ClassLoader? #

We know that Java Program runs on [Java Virtual Machine][] (JVM). When we compile a Java Class, it transforms it in the form of bytecode that is platform and machine independent compiled program and store it as a .class file. After that when we try to use a Class, Java ClassLoader loads that class into memory.

There are three types of built-in ClassLoader in Java:

1.  **Bootstrap Class Loader** – It loads JDK internal classes, typically loads rt.jar and other core classes for example java.lang.\* package classes
2.  **Extensions Class Loader** – It loads classes from the JDK extensions directory, usually $JAVA\_HOME/lib/ext directory.
3.  **System Class Loader** – It loads classes from the current classpath that can be set while invoking a program using -cp or -classpath command line options.

Java ClassLoader are hierarchical and whenever a request is raised to load a class, it delegates it to its parent and in this way uniqueness is maintained in the runtime environment. If the parent class loader doesn’t find the class then the class loader itself tries to load the class.

Lets understand this by executing the below java program:

`ClassLoaderTest.java`

package chimomo;
    
    
    /**
     * @author Chimomo
     */
    public class ClassLoaderTest {
    
    
        public static void main(String[] args) {
            System.out.println("class loader for HashMap: " + java.util.HashMap.class.getClassLoader());
            System.out.println("class loader for DNSNameService: " + sun.net.spi.nameservice.dns.DNSNameService.class.getClassLoader());
            System.out.println("class loader for this class: " + ClassLoaderTest.class.getClassLoader());
        }
    
    
    }

Output of the above java classloader example program is:

class loader for HashMap: null
    class loader for DNSNameService: sun.misc.Launcher$ExtClassLoader@42a57993
    class loader for this class: sun.misc.Launcher$AppClassLoader@15db9742

As you can see that java.util.HashMap ClassLoader is coming as null that reflects Bootstrap ClassLoader whereas DNSNameService ClassLoader is ExtClassLoader. Since the class itself is in CLASSPATH, System ClassLoader loads it.

When we are trying to load HashMap, our System ClassLoader delegates it to the Extension ClassLoader, which in turns delegates it to Bootstrap ClassLoader that found the class and load it in JVM.

The same process is followed for DNSNameService class but Bootstrap ClassLoader is not able to locate it since its in $JAVA\_HOME/lib/ext/dnsns.jar and hence gets loaded by Extensions Class Loader.

One more important point to note is that Classes loaded by a child class loader have visibility into classes loaded by its parent class loaders. So classes loaded by System ClassLoader have visibility into classes loaded by Extensions and Bootstrap ClassLoader.

If there are sibling class loaders then they can’t access classes loaded by each other.

# Why write a Custom ClassLoader in Java? #

Java default ClassLoader can load files from local file system that is good enough for most of the cases. But if you are expecting a class at the runtime or from FTP server or via third party web service at the time of loading the class then you have to extend the existing class loader. For example, AppletViewers load the classes from remote web server.

# How does Java ClassLoader Work? #

When JVM requests for a class, it invokes `loadClass` function of the ClassLoader by passing the fully classified name of the Class.

loadClass function calls for `findLoadedClass()` method to check that the class has been already loaded or not. It’s required to avoid loading the class multiple times.

If the Class is not already loaded then it will delegate the request to parent ClassLoader to load the class.

If the parent ClassLoader is not finding the Class then it will invoke findClass() method to look for the classes in the file system.

# Java Custom ClassLoader #

We will create our own ClassLoader by extending ClassLoader class and overriding loadClass(String name) method. If the name will start from com.journaldev i.e our sample classes package then we will load it using our own class loader or else we will invoke the parent ClassLoader loadClass() method to load the class.

The project structure will be like the below image:

![SouthEast][]

**CCLoader.java**: This is our custom class loader with below methods.

1.  `private byte[] loadClassFileData(String name)`:
    
    This method will read the class file from file system to byte array.
2.  `private Class getClass(String name)`
    
    This method will call the loadClassFileData() function and by invoking the parent defineClass() method, it will generate the Class and return it.
3.  `public Class loadClass(String name)`:
    
    This method is responsible for loading the Class. If the class name starts with com.journaldev (Our sample classes) then it will load it using getClass() method or else it will invoke the parent loadClass function to load it.
4.  `public CCLoader(ClassLoader parent)`:
    
    This is the constructor which is responsible for setting the parent ClassLoader.

`CCLoader.java`

package chimomo;
    
    
    import java.io.DataInputStream;
    import java.io.File;
    import java.io.IOException;
    import java.io.InputStream;
    
    
    /**
     * Our Custom Class Loader to load the classes. Any class in the chimomo package
     * will be loaded using this ClassLoader. For other classes, it will delegate
     * the request to its Parent ClassLoader.
     *
     * @author Chimomo
     */
    public class CCLoader extends ClassLoader {
    
    
        /**
         * This constructor is used to set the parent ClassLoader
         *
         * @param parent The parent class loader
         */
        public CCLoader(ClassLoader parent) {
            super(parent);
        }
    
    
        /**
         * Loads the class from the file system. The class file should be located in
         * the file system. The name should be relative to get the file location
         *
         * @param name Fully Classified name of class, for example chimomo.Foo
         */
        private Class getClass(String name) throws ClassNotFoundException {
            String file = name.replace('.', File.separatorChar) + ".class";
            byte[] b;
            try {
                // This loads the byte code data from the file  
                b = loadClassFileData(file);
                // defineClass is inherited from the ClassLoader class  
                // that converts byte array into a Class. defineClass is Final  
                // so we cannot override it  
                Class c = defineClass(name, b, 0, b.length);
                resolveClass(c);
                return c;
            } catch (IOException e) {
                e.printStackTrace();
                return null;
            }
        }
    
    
        /**
         * Every request for a class passes through this method. If the class is in
         * chimomo package, we will use this class loader or else delegate the
         * request to parent class loader.
         *
         *
         * @param name Full class name
         * @return The loaded class
         * @throws java.lang.ClassNotFoundException
         */
        @Override
        public Class loadClass(String name) throws ClassNotFoundException {
            System.out.println("Loading Class '" + name + "'");
            if (name.startsWith("chimomo")) {
                System.out.println("Loading Class using CCLoader");
                return getClass(name);
            }
            return super.loadClass(name);
        }
    
    
        /**
         * Reads the file (.class) into a byte array. The file should be accessible
         * as a resource and make sure that it's not in CLASSPATH to avoid any
         * confusion.
         *
         * @param name File name
         * @return Byte array read from the file
         * @throws IOException if any exception comes in reading the file
         */
        private byte[] loadClassFileData(String name) throws IOException {
            InputStream stream = getClass().getClassLoader().getResourceAsStream(name);
            int size = stream.available();
            byte buff[] = new byte[size];
            try (DataInputStream in = new DataInputStream(stream)) {
                in.readFully(buff);
            }
            return buff;
        }
    }

**CCRun.java**:

This is our test class with main function where we are creating object of our ClassLoader and load sample classes using its loadClass method. After loading the Class, we are using [Java Reflection API][] to invoke its methods.

`CCRun.java`

package chimomo;
    
    
    import java.lang.reflect.Method;
    
    
    /**
     * @author Chimomo
     */
    public class CCRun {
    
    
        public static void main(String args[]) throws Exception {
            String progClass = args[0];
            String progArgs[] = new String[args.length - 1];
            System.arraycopy(args, 1, progArgs, 0, progArgs.length);
    
    
            CCLoader ccl = new CCLoader(CCRun.class.getClassLoader());
            Class clas = ccl.loadClass(progClass);
            Class mainArgType[] = {(new String[0]).getClass()};
            Method main = clas.getMethod("main", mainArgType);
            Object argsArray[] = {progArgs};
            main.invoke(null, argsArray);
    
    
            // Below method is used to check that the Foo is getting loaded  
            // by our custom class loader i.e CCLoader  
            Method printCL = clas.getMethod("printCL", (Class[]) null);
            printCL.invoke(null, new Object[0]);
        }
    
    
    }

**Foo.java and Bar.java**:

These are our test classes that is getting loaded by our custom classloader. They also have a printCL() method that is getting invoked to print the ClassLoader that has loaded the Class. Foo class will be loaded by our custom class loader which in turn uses Bar class, so Bar class will also be loaded by our custom class loader.

`Foo.java`

package chimomo;
    
    
    /**
     * @author Chimomo
     */
    public class Foo {
    
    
        static public void main(String args[]) throws Exception {
            System.out.println("Foo Constructor >>> " + args[0] + " " + args[1]);
            Bar bar = new Bar(args[0], args[1]);
            bar.printCL();
        }
    
    
        public static void printCL() {
            System.out.println("Foo ClassLoader: " + Foo.class.getClassLoader());
        }
    }

Bar.java

package chimomo;
    
    
    /**
     * @author Chimomo
     */
    public class Bar {
    
    
        public Bar(String a, String b) {
            System.out.println("Bar Constructor >>> " + a + " " + b);
        }
    
    
        public void printCL() {
            System.out.println("Bar ClassLoader: " + Bar.class.getClassLoader());
        }
    }

# Java Custom ClassLoader Execution Steps #

First of all we will compile all the classes through command line. After that we will run CCRun class by passing three arguments. The first argument is the fully classified name for Foo class that will get loaded by our class loader. Other two arguments are passed along to the Foo class main function and Bar constructor. The execution steps with output will be like below.

Chimomo$ java chimomo.CCRun chimomo.Foo 1212 1313                                                                    
    Loading Class 'chimomo.Foo'
    Loading Class using CCLoader
    Loading Class 'java.lang.Object'
    Loading Class 'java.lang.String'
    Loading Class 'java.lang.Exception'
    Loading Class 'java.lang.System'
    Loading Class 'java.lang.StringBuilder'
    Loading Class 'java.io.PrintStream'
    Foo Constructor >>> 1212 1313
    Loading Class 'chimomo.Bar'
    Loading Class using CCLoader
    Bar Constructor >>> 1212 1313
    Loading Class 'java.lang.Class'
    Bar ClassLoader: chimomo.CCLoader@5c647e05
    Foo ClassLoader: chimomo.CCLoader@5c647e05

If you look into the output carefully, first its trying to load chimomo.Foo class but since its extending java.lang.Object class, its trying to load it first and the request it coming to CCLoader loadClass method that is delegating it to the parent class. So the parent class loaders are loading the Object, String and other java classes. Our ClassLoader is only loading Foo and Bar class from the file system that is getting clear when we invoke their printCL() function.

Note that we can change the loadClassFileData() functionality to read the byte array from FTP Server or by invoking any third party service to get the class byte array on the fly.

I hope that the article will be useful in understanding Java ClassLoader working and how we can extend it to do a lot more that just taking it from file system.

[http_www.captainbed.net]: http://www.captainbed.net/troubleshooter
[Java Virtual Machine]: https://www.journaldev.com/546/difference-jdk-vs-jre-vs-jvm
[SouthEast]: /images/20220602/49ca2c6545534b40a4492bd1ebda3b4b.png
[Java Reflection API]: https://www.journaldev.com/1789/java-reflection-example-tutorial