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 contents
public 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 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: ");
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);
}
}
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> <modelVersion>4.0.0</modelVersion>
<groupId>com.weihua.blockchains</groupId>
<artifactId>blackchain</artifactId>
<version>0.0.1-SNAPSHOT</version>
<packaging>jar</packaging>
<name>blackchain</name>
<url>http://maven.apache.org</url>
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
</properties>
<dependencies>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>3.8.1</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>com.google.code.gson</groupId>
<artifactId>gson</artifactId>
<version>2.1</version>
</dependency>
</dependencies>
</project>
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 } ]
