java instance understands the concept of blockchain

   The core of blockchain is decentralized storage. Traditional database solutions, including relational database and non relational database, belong to centralized storage. Decentralized storage means that there is no data center, and each data node contains the information of the previous data node.

         An example is given to understand the data storage form of blockchain:

package com.weihua.blockchains.blackchain;

import java.util.Date;

public class BlockMan {

public String hash;
public String previousHash; 
private String data; //our data will be a simple message.
private long timeStamp; //as number of milliseconds since 1/1/1970.
private int nonce;

//Block Constructor.  
public BlockMan(String data,String previousHash ) { = data;
    this.previousHash = previousHash;
    this.timeStamp = new Date().getTime();

    this.hash = calculateHash(); //Making sure we do this after we set the other values.

//Calculate new hash based on blocks contents
public String calculateHash() {
    String calculatedhash = StringUtil.applySha256( 
            previousHash +
            Long.toString(timeStamp) +
            Integer.toString(nonce) + 
    return calculatedhash;

//Increases nonce value until hash target is reached.
public void mineBlock(int difficulty) {
    String target = StringUtil.getDificultyString(difficulty); //Create a string with difficulty * "0" 
    while(!hash.substring( 0, difficulty).equals(target)) {
        nonce ++;
        hash = calculateHash();
    System.out.println("Block Mined!!! : " + hash);


package com.weihua.blockchains.blackchain;


public class StringUtil {

//Applies Sha256 to a string and returns the result. 
public static String applySha256(String input){

    try {
        MessageDigest digest = MessageDigest.getInstance("SHA-256");

        //Applies sha256 to our input, 
        byte[] hash = digest.digest(input.getBytes("UTF-8"));

        StringBuffer hexString = new StringBuffer(); // This will contain hash as hexidecimal
        for (int i = 0; i < hash.length; i++) {
            String hex = Integer.toHexString(0xff & hash[i]);
            if(hex.length() == 1) hexString.append('0');
        return hexString.toString();
    catch(Exception e) {
        throw new RuntimeException(e);

//Short hand helper to turn Object into a json string
public static String getJson(Object o) {
    return new GsonBuilder().setPrettyPrinting().create().toJson(o);

//Returns difficulty string target, to compare to hash. eg difficulty of 5 will return "00000"  
public static String getDificultyString(int difficulty) {
    return new String(new char[difficulty]).replace('\0', '0');


package com.weihua.blockchains.blackchain;
import java.util.ArrayList;

public class TestBlockChain {

public static ArrayList<BlockMan> blockchain = new ArrayList<BlockMan>();
public static int difficulty = 5;

public static void main(String[] args) {    
    //add our blocks to the blockchain ArrayList:

    System.out.println("Trying to Mine block 1... ");
    addBlock(new BlockMan("Hi im the first block", "0"));

    System.out.println("Trying to Mine block 2... ");
    addBlock(new BlockMan("Yo im the second block",blockchain.get(blockchain.size()-1).hash));

    System.out.println("Trying to Mine block 3... ");
    addBlock(new BlockMan("Hey im the third block",blockchain.get(blockchain.size()-1).hash));  

    System.out.println("\nBlockchain is Valid: " + isChainValid());

    String blockchainJson = StringUtil.getJson(blockchain);
    System.out.println("\nThe block chain: ");

public static Boolean isChainValid() {
    BlockMan currentBlock; 
    BlockMan previousBlock;
    String hashTarget = new String(new char[difficulty]).replace('\0', '0');

    //loop through blockchain to check hashes:
    for(int i=1; i < blockchain.size(); i++) {
        currentBlock = blockchain.get(i);
        previousBlock = blockchain.get(i-1);
        //compare registered hash and calculated hash:
        if(!currentBlock.hash.equals(currentBlock.calculateHash()) ){
            System.out.println("Current Hashes not equal");         
            return false;
        //compare previous hash and registered previous hash
        if(!previousBlock.hash.equals(currentBlock.previousHash) ) {
            System.out.println("Previous Hashes not equal");
            return false;
        //check if hash is solved
        if(!currentBlock.hash.substring( 0, difficulty).equals(hashTarget)) {
            System.out.println("This block hasn't been mined");
            return false;

    return true;

public static void addBlock(BlockMan newBlock) {


<project xmlns="" xmlns:xsi=""








Run to see the output:
Trying to Mine block 1...
Block Mined!!! : 000005b9947556fa4b14d24349728a58418c1517f8073d10a37ba3daf0ceee9c
Trying to Mine block 2...
Block Mined!!! : 00000015d3f53bd8545a3a9586aaef26abe26d78fce2da6584aac025c60c21e3
Trying to Mine block 3...
Block Mined!!! : 000000d224711a1e308e98c317cc7c900c7f4196d585266262b54e42288da65b

Blockchain is Valid: true

The block chain:
"hash": "000005b9947556fa4b14d24349728a58418c1517f8073d10a37ba3daf0ceee9c",
"previousHash": "0",
"data": "Hi im the first block",
"timeStamp": 1557928459188,
"nonce": 347042
"hash": "00000015d3f53bd8545a3a9586aaef26abe26d78fce2da6584aac025c60c21e3",
"previousHash": "000005b9947556fa4b14d24349728a58418c1517f8073d10a37ba3daf0ceee9c",
"data": "Yo im the second block",
"timeStamp": 1557928460017,
"nonce": 1394831
"hash": "000000d224711a1e308e98c317cc7c900c7f4196d585266262b54e42288da65b",
"previousHash": "00000015d3f53bd8545a3a9586aaef26abe26d78fce2da6584aac025c60c21e3",
"data": "Hey im the third block",
"timeStamp": 1557928462949,
"nonce": 81919

Tags: Programming Blockchain Maven Apache Database

Posted on Sun, 10 Nov 2019 16:38:52 -0500 by karan23424