CS101: Introduction to Computer Science I

External Java packages (such as Servlet, MySQL Connector/J, JOGL, JUnit) are often distributed in JAR files (Java Archive - a single-file package of many Java classes), with possibly Native Libraries (".lib " and ".dll" in Windows, or ".a " and ".so " in Linux/Mac).

External JAR Files (".jar")

If external JAR files are not properly included:

• During the compilation, you will receive compilation error "cannot find symbol" on classes belonging to the external packages.
• During execution, you will get a runtime error "Could not find or load main class xxx" or "NoClassDefFoundError ".

To include external JAR files, you can either:

1. Copy all the JAR files of the external packages to the Java's Extension Directories (NOT applicable to JDK 9).
   • For Windows, the JDK extension directory is located at "<JAVA_HOME>\jre\lib\ext" (e.g., "c:\Program Files\Java\jdk1.8.0_xx\jre\lib\ext ").
   • For Mac, the JDK extension directories are "/Library/Java/Extensions" and "/System/Library/Java/Extensions ".
   • For Ubuntu, the JDK extension directories are "<JAVA_HOME>/jre/lib/ext" (e.g., "/usr/user/java/jdk1.8.0_xx/jre/lib/ext ") and "/usr/java/packages/lib/ext".
   The location of JDK's extension directories is kept in Java's System Property "java.ext.dirs". You can print its contents via System.out.println(System.getProperty("java.ext.dirs")) .
2. You can also include all the JAR files in the CLASSPATH environment variable. CLASSPATH  may contain directories (of Java classes) or JAR files (single-file archive of Java classes). If you set the CLASSPATH , you must also include the current directory  (denoted as ".").
   • For Windows, set the CLASSPATH in Control Panel ⇒ System ⇒ Advanced system settings ⇒ Advanced ⇒ Environment Variables ⇒ System Variables ⇒ New ⇒ In "Variable name", enter "CLASSPATH " ⇒ In "Variable value", enter ".;path1\xxx.jar;path2\yyy.jar ", where the entries are separated by a semi-colon (;).
   • For Linux and Mac OS: Edit ~/.profile or ~/.bash_profile  (or /etc/profile for system-wide setting) to include the following line at the end of the file:

   export CLASSPATH=.:path1/xxx.jar:path2/yyy.jar

   The entries are separated by colon (:).
3. You can also set the CLASSPATH in the javac /java command-line via the option -cp <paths>  (or -classpath <paths> ), for example.
   

// For Windows
// Compile Java source code
javac -cp .;path1\xxx.jar;path2\yyy.jar ClassName.java
// Run Java class
java -cp .;path1\xxx.jar;path2\yyy.jar ClassName
 
// For Mac OS X and Ubuntu
// Compile Java source code
javac -cp .:path1/xxx.jar:path2/yyy.jar ClassName.java
// Run Java class
java -cp .:path1/xxx.jar:path2/yyy.jar ClassName

External Native Libraries (".lib", ".dll", ".a", ".so")

Some external package may provide static or shared native libraries in the form of ".lib" (Windows' static LIBrary), ".dll " (Windows' Dynamically Link Library), ".a" (Unix's static (Archive) library), or ".so " (Unix's Shared Object library).

Native Libraries are to be kept in a directory accessible via JRE's Property "java.library.path", which normally but not necessarily  includes all the directories in the PATH environment variable.

Native libraries are not involved in the compilation. But if they are not properly included during runtime time, you will get a runtime error "java.lang.UnsatisfiedLinkError: no xxx in java.library.path ".

To include external native libraries:

1. Copy the native libraries into a system library directory, e.g., c:\windows\system32 (Windows), /usr/lib  or /usr/local/lib (Mac OS X / Unix). You can verify that the directory is included in Java's System Property "java.library.path ", via System.out.println(System.getProperty("java.library.path")).
2. You can also set the native library path via the java's command-line option -Djava.library.path=xxx , for example,

   // Run a Java program
   java -Djava.library.path=xxx ClassName
   

Eclipse/NetBeans

Using an IDE can greatly simplifies inclusion of external packages. Read "Eclipse How-To " or "NetBeans How-To ".