🍵 Java

Most of these notes are from the book Java Projects by Peter Verhas.

Maven build lifecycle targets

  • validate - validate the project is correct and all necessary information is available
  • compile - compile the source code of the project
  • test - test the compiled source code using a suitable unit testing framework. These tests should not require the code be packaged or deployed
  • package - take the compiled code and package it in its distributable format, such as a JAR.
  • verify - run any checks on results of integration tests to ensure quality criteria are met
  • install - install the package into the local repository, for use as a dependency in other projects locally
  • deploy - done in the build environment, copies the final package to the remote repository for sharing with other developers and projects.

From the official documentation.

JConsole over SSH with SOCKS Proxy

This is from here: Create the SSH socks proxy locally on some free port (e.g. 7777):

ssh -fN -D 7777 user@firewalled-host

Run JConsole by specifying the SOCKS proxy (e.g. localhost:7777) and the address for the JMX server (e.g. localhost:2147)

jconsole -J-DsocksProxyHost=localhost -J-DsocksProxyPort=7777 service:jmx:rmi:///jndi/rmi://localhost:2147/jmxrmi -J-DsocksNonProxyHosts=

JShell

  • /list -start - shows modules imported at startup.
  • /edit <number> - edit that line in a new window.
  • /set editor "vi" - use vi instead of the default graphical edit pad.
  • /save abc.java - save current buffer to file.
  • /load abc.java - load from file into shell.
  • /-1 - execute last snippet.
  • /1 - execute first snippet.
  • /drop N - drop Nth snippet.
  • /vars - show only variables that were defined in snippets.
  • /types - show only classes that were defined in snippets.

Javap

  • javap TestDecompile.class - decompile .class file to human readable format. Does not show content of methods though.

  • javap -c TestDecompile.class - show jvm bytecode in human readable -form, including methods.

Jar files

These are zip files that have a META-INF folder with a Manifest.mf file inside.

To create a jar file:

jar -cf hello.jar HelloWorld.class

And to run:

java -cp hello.jar HelloWorld

To see contents of a jar:

jar -tf hello.jar

Runtime/debugging

  • jps - Shows all runnning java processes.

  • jvisualvm - If you have it, it shows the java processes on the system with details on threads, profiler etc.

  • -Xagentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=7896 Use these args to start a process in debugging mode.

    • agentlib=jdwp - Load the jdwp agent, for debugging.

    • transport=dt_socket - for connecting a debugging client over the network.

    • server=y - this one is the server half of the debugging.

    • suspend=y - don't start executing until a client debugger attaches.

    • address=7896 - port we listen on.

Maven

  • Expected directory structure: Java files are in src/main/java as well as src/test/java. Resource files are under src/main/resources and src/test/resources.

  • mvn archetype:generate: Generates a skeleton of a project based on your inputs (package name, versioning, project name, etc).

  • Edit pom.xml and set the jdk version there.. the default may be old. Java 9 onwaards doesn't generate the .class format. You should have this added to the default .xml file:

<build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>3.8.0</version>
        <configuration>
            <source>1.11</source>
            <target>1.11</target>
            <release>11</release>
        </configuration>
      </plugin>
    </plugins>
  </build>
  • mvn package - compile, test, bundle.

Interfaces

  • All fields (i.e. class variables) default to static final. All methods default to public.

Field types

  • static - One instance per class, not per object.

  • final - Initialized exactly once, either where declared or

  • somewhere in the constructor. Immutable after that.

  • transient - Do not serialize/deserialize this field.

  • volatile - Treat as a field that may be accessed by multiple threads. If not volatile, the value may be stored in a processor cache or registry for faster access.

Things to find out later

  • See how to create a call graph in eclipse.

  • final keyword?

    • Answer: when used with a variable, it (a) blocks reassignment, and (b) limits existence to that block. e.g. a final String tmp = orig inside a for loop cannot be reassigned with a different value. The value does not exist outside the block it is defined in. Good practice to declare this often as it means a variable is guaranteed not to change in value.