java 实例理解区块链的概念

   区块链的核心是去中心化的存储，传统的数据库解决方案，包括关系型数据库，非关系型数据库，都是属于中心化的存储方式。去中心化的存储，就是数据没有中心，并且每个数据节点都包含了上一个数据节点的信息。         通过一个实例来理解区块链的数据存储形式：

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 ) {    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 contentspublic String calculateHash() {    String calculatedhash = StringUtil.applySha256(             previousHash +            Long.toString(timeStamp) +            Integer.toString(nonce) +             data             );    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;
import java.security.MessageDigest;

import com.google.gson.GsonBuilder;

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');            hexString.append(hex);        }        return hexString.toString();    }    catch(Exception e) {        throw new RuntimeException(e);    }}//Short hand helper to turn Object into a json stringpublic 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 blockchain = new ArrayList();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: ");    System.out.println(blockchainJson);}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) {    newBlock.mineBlock(difficulty);    blockchain.add(newBlock);}

}

xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
4.0.0

com.weihua.blockchainsblackchain0.0.1-SNAPSHOTjarblackchainhttp://maven.apache.org    UTF-8            junit        junit        3.8.1        test                com.google.code.gson        gson        2.1    

运行起来看下输出：
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
}
]