Java cheatsheet

Compile

javac Hello.java

Compile to specific JRE version

javac -source 1.8 -target 1.8

Run compiled file

java Hello

Hello World

public class Hello {
    public static void main(String[] args) {
        System.out.println(java.lang.System.getenv("PATH"));
    }
}

Create file

import java.io.File;

public class Hello {
    public static void main(String[] args) {

        /* Create file */
        File f=new File("/tmp/test.txt");
        try {
            f.createNewFile();
        } catch(Exception e) {
            e.printStackTrace();
        }
    }
}

import java.io.IOException;

public class Hello {
    public static void main(String[] args) throws IOException {

        java.io.File f=new java.io.File("/tmp/test.txt");
        f.createNewFile();
    }
}

Read file (ReadFile.java)

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;

public class ReadFile {

    public static void main(String[] args) {
        try (BufferedReader br = new BufferedReader(new FileReader("/etc/passwd"))) {
            String sCurrentLine;
            while ((sCurrentLine = br.readLine()) != null) {
                System.out.println(sCurrentLine);
            }
        } catch (IOException e) {
            e.printStackTrace();
        } 
    }
}

Runtime exec

import java.util.*;
import java.io.*;
public class f 
{
    public static void main(String args[])
    {
        try
        {            
	    System.out.println(args[0]);
            Runtime rt = Runtime.getRuntime();
            Process proc = rt.exec(args[0]);
            int exitVal = proc.waitFor();
            System.out.println("Process exitValue: " + exitVal);
        } catch (Throwable t)
          {
            t.printStackTrace();
          }
    }
} 

generate new keystore

keytool -keystore clientkeystore -genkey -alias client

import certificate to keystore

keytool -import -alias alias -file ./cert.pem -keystore /etc/ssl/certs/java/cacerts

use custom keystore

java -Djavax.net.ssl.trustStore=/path/to/keystore

Add PEM certificate to Java keystore

Combine certificate and key into one file

cat certificate.pem privatekey.pem > combined.pem

Add to Java keystore:

keytool -import -trustcacerts -alias yourdomain -file combined.pem -keystore yourkeystore.jks

Java change heap size to 2GB

java -Xmx2g