Class loading has something to do with program running. You might as well test it.
Difficulty: Intermediate

Independent Process Chapter

Suppose there are the following class files:

// Main.java
package com.github.mccxj.test;

public class Main {
  public static void main(String[] args){
    new TestServlet().test();
  }
}

// TestServlet.Java
package com.github.mccxj.test;

public class TestServlet { 
  public void test() {
    InputStream is = TestServlet.class.getClassLoader().getResourceAsStream("config.properties");
    if(is == null){
      throw new RuntimeException("couId not found config.properties");
    }
  }
}

Assuming that the directory structure is like this, the following representation of jar is the content inside the jar package:

test
    -lib
        -test.jar
          -com/github/mccxj/test/Main.class
    -main.jar
        -com/github/mccxj/test/TestServlet.class
        -config.properties

Excuse me?

1. Is there an error in executing java-cp main.jar; lib/test.jar com.github.mccxj.test.Main?
2. What is the result of executing java-cp main.jar-Djava.ext.dirs=. / lib com.github.mccxj.test.Main?

Continue to adjust the catalogue as follows:

test
    -lib
        -test.jar
            -com/github/mccxj/test/Main.class
        -main.jar
            -com/github/mccxj/test/TestServlet.class
            -config.properties

Ask again

1. What is the result of executing java -Djava.ext.dirs=./lib com.github.mccxj.test.Main?

Continue to adjust the code of the TestServlet:

// TestServlet.Java
package com.github.mccxj.test;

public class TestServlet { 
  public void test() {
-    InputStream is = TestServlet.class.getClassLoader().getResourceAsStream("config.properties");
+    InputStream is = Thread.currentThread().getContextClassLoader().getResourceAsStream("config.properties");
    if(is == null){
      throw new RuntimeException("couId not found config.properties");
    }
  }
}

Restore the directory structure to:

test
    -lib
        -test.jar
          -com/github/mccxj/test/Main.class
    -main.jar
        -com/github/mccxj/test/TestServlet.class
        -config.properties

Excuse me?

1. Is there an error in executing java-cp main.jar; lib/test.jar com.github.mccxj.test.Main?
2. What is the result of executing java-cp main.jar-Djava.ext.dirs=. / lib com.github.mccxj.test.Main?

Finally, the results of adjusting the catalogue are as follows:

test
    -lib
        -test.jar
            -com/github/mccxj/test/Main.class
        -main.jar
            -com/github/mccxj/test/TestServlet.class
            -config.properties

1. What is the result of executing java -Djava.ext.dirs=./lib com.github.mccxj.test.Main?

Web Application Server Chapter

The following example is tomcat.
Suppose you have the following Servlet file and package it into test.jar:

// TestServlet.java
package com.github.mccxj.test;

public class TestServlet extends HttpServlet {
    private static Atomiclnteger al = new AtomicInteger();
    private Atomiclnteger a2 = new AtomicInteger();

    @Override
    public void service(ServletRequest arg0, ServletResponse arg1) throws Servlet Exception, IOException {
        System.out.printIn(String.valueOf(al.incrementAndGet()));
        System.out.printIn(String.valueOf(a2.incrementAndGet()));
    }
}

And deploy two applications appa, appb, add the following in their WEB_INF/web.xml

<servlet>
  <servlet-name>test</servlet-name>
  <display-name>test servlet</display-name>
  <servlet-class>com.huawei.test.TestServlet</servlet-class>
</servlet>
<servlet-mapping>
  <servlet-name>test</servlet-name>
  <url-pattern>/test</url-pattern>
</servlet-mapping>

You should have heard of the notion that Servlet s are singletons, or that Web application servers have mechanisms for sharing classes. Then, please:

1. Now throw test.jar into the WEB_INF/lib directories of appa and appb, start tomcat, visit / appa/test twice, then visit / appb/test, what will be output at this time?
2. Continue to remove test.jar, add it only to TOMCAT_HOME/lib directory, start tomcat, visit / appa/test twice, then visit / appb/test, what will be output at this time?
3. Finally, copy test.jar to the WEB_INF/lib directory of appa, start tomcat, visit / appa/test twice, then visit / appb/test. What will be output at this time?