【区块链】使用JAV简易A模拟创建区块链及挖矿

想法：
1.创建区块链。区块链是由一个个区块组成的链表，每个区块包含前一个区块的哈希和本身区块的哈希。每个区块还包含数据，创建事件，nonce等信息。
2.使用SHA256生成哈希值。
3.挖矿。找到哈希值为前diffcult位数0的nonce值。
代码：

import java.util.Date;
import java.util.ArrayList;
import com.google.gson.GsonBuilder;
import java.security.MessageDigest;

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');
                hexString.append(hex);
            }
            return hexString.toString();
        } catch (Exception e) {
    
    
            throw new RuntimeException(e);
        }
    }
}
class Block {
    
    
    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 Block(String data, String previousHash) {
    
    
        this.data = 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) +
                        data
        );
        return calculatedhash;
    }

    public void mineBlock(int difficulty) {
    
    
        String target = new String(new char[difficulty]).replace('\0', '0'); //Create a string with difficulty * "0"
        //System.out.println(target);
        while (!hash.substring(0, difficulty).equals(target)) {
    
    
            nonce++;
            hash = calculateHash();
        }
        System.out.println("Block Mined!!! : " + hash);
    }
}


public class main{
    
    
    public static int difficulty = 5;
    public static ArrayList<Block> blockchain = new ArrayList<Block>();
    public static void main(String[] args) {
    
    
        //add our blocks to the blockchain ArrayList:

        blockchain.add(new Block("Hi im the first block", "0"));
        System.out.println("Trying to Mine block 1... ");
        blockchain.get(0).mineBlock(difficulty);

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

        blockchain.add(new Block("Hey im the third block", blockchain.get(blockchain.size() - 1).hash));
        System.out.println("Trying to Mine block 3... ");
        blockchain.get(2).mineBlock(difficulty);

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

        String blockchainJson = new GsonBuilder().setPrettyPrinting().create().toJson(blockchain);
        System.out.println("\nThe block chain: ");
        System.out.println(blockchainJson);
    }

    public static Boolean isChainValid() {
    
    
        Block currentBlock;
        Block 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;
    }
}