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Adding source code for the article tracked under BAEL-3232. (eugenp#7657)
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‎java-blockchain/.gitignore‎

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*.class
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0.*
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#folders#
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/target
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/neoDb*
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/data
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/src/main/webapp/WEB-INF/classes
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*/META-INF/*
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.resourceCache
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# Packaged files #
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*.jar
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*.war
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*.ear
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# Files generated by integration tests
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*.txt
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backup-pom.xml
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/bin/
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/temp
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#IntelliJ specific
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.idea/
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*.iml

‎java-blockchain/README.md‎

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=========
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## Basic Implementation of Blockchian in Java
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### Relevant Articles:
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- []()

‎java-blockchain/pom.xml‎

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<project xmlns="http://maven.apache.org/POM/4.0.0"
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xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
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<modelVersion>4.0.0</modelVersion>
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<groupId>com.baeldung.blockchain</groupId>
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<artifactId>java-blockchain</artifactId>
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<version>0.1.0-SNAPSHOT</version>
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<name>java-blockchain</name>
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<packaging>jar</packaging>
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<parent>
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<groupId>com.baeldung</groupId>
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<artifactId>parent-java</artifactId>
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<version>0.0.1-SNAPSHOT</version>
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<relativePath>../parent-java</relativePath>
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</parent>
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<build>
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<finalName>java-blockchain</finalName>
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<resources>
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<resource>
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<directory>src/main/resources</directory>
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<filtering>true</filtering>
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</resource>
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</resources>
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<plugins>
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<plugin>
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<groupId>org.apache.maven.plugins</groupId>
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<artifactId>maven-compiler-plugin</artifactId>
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<version>${maven-compiler-plugin.version}</version>
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<configuration>
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<source>${maven.compiler.source}</source>
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<target>${maven.compiler.target}</target>
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</configuration>
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</plugin>
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</plugins>
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</build>
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<properties>
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<maven.compiler.source>1.8</maven.compiler.source>
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<maven.compiler.target>1.8</maven.compiler.target>
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</properties>
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</project>

‎java-blockchain/src/main/java/com/baeldung/blockchain/Block.java‎

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package com.baeldung.blockchain;
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import java.io.UnsupportedEncodingException;
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import java.security.MessageDigest;
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import java.security.NoSuchAlgorithmException;
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import java.util.Date;
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import java.util.logging.Level;
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import java.util.logging.Logger;
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public class Block {
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private static Logger logger = Logger.getLogger(Block.class.getName());
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private String hash;
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private String previousHash;
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private String data;
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private long timeStamp;
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private int nonce;
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public Block(String data, String previousHash) {
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this.data = data;
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this.previousHash = previousHash;
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this.timeStamp = new Date().getTime();
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this.hash = calculateBlockHash();
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}
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public String mineBlock(int prefix) {
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String prefixString = new String(new char[prefix]).replace('\0', '0');
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while (!hash.substring(0, prefix)
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.equals(prefixString)) {
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nonce++;
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hash = calculateBlockHash();
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}
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return hash;
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}
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public String calculateBlockHash() {
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String dataToHash = previousHash + Long.toString(timeStamp) + Integer.toString(nonce) + data;
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MessageDigest digest = null;
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byte[] bytes = null;
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try {
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digest = MessageDigest.getInstance("SHA-256");
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bytes = digest.digest(dataToHash.getBytes("UTF-8"));
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} catch (NoSuchAlgorithmException | UnsupportedEncodingException ex) {
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logger.log(Level.SEVERE, ex.getMessage());
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}
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StringBuffer buffer = new StringBuffer();
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for (byte b : bytes) {
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buffer.append(String.format("%02x", b));
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}
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return buffer.toString();
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}
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public String getHash() {
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return this.hash;
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}
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public String getPreviousHash() {
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return this.previousHash;
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}
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public void setData(String data) {
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this.data = data;
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}
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}

‎java-blockchain/src/main/resources/logback.xml‎

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<?xml version="1.0" encoding="UTF-8"?>
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<configuration>
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<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
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<encoder>
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<pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n
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</pattern>
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</encoder>
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</appender>
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<root level="INFO">
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<appender-ref ref="STDOUT" />
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</root>
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</configuration>

‎java-blockchain/src/test/java/com/baeldung/blockchain/BlockchainUnitTest.java‎

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package com.baeldung.blockchain;
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import static org.junit.jupiter.api.Assertions.assertTrue;
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import java.util.ArrayList;
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import java.util.List;
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import org.junit.AfterClass;
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import org.junit.BeforeClass;
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import org.junit.Test;
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public class BlockchainUnitTest {
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public static List<Block> blockchain = new ArrayList<Block>();
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public static int prefix = 4;
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public static String prefixString = new String(new char[prefix]).replace('\0', '0');
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@BeforeClass
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public static void setUp() {
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Block genesisBlock = new Block("The is the Genesis Block.", "0");
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genesisBlock.mineBlock(prefix);
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blockchain.add(genesisBlock);
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Block firstBlock = new Block("The is the First Block.", genesisBlock.getHash());
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firstBlock.mineBlock(prefix);
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blockchain.add(firstBlock);
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}
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@Test
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public void givenBlockchain_whenNewBlockAdded_thenSuccess() {
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Block newBlock = new Block("The is a New Block.", blockchain.get(blockchain.size() - 1)
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.getHash());
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newBlock.mineBlock(prefix);
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assertTrue(newBlock.getHash()
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.substring(0, prefix)
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.equals(prefixString));
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blockchain.add(newBlock);
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}
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@Test
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public void givenBlockchain_whenValidated_thenSuccess() {
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boolean flag = true;
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for (int i = 0; i < blockchain.size(); i++) {
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String previousHash = i == 0 ? "0"
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: blockchain.get(i - 1)
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.getHash();
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flag = blockchain.get(i)
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.getHash()
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.equals(blockchain.get(i)
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.calculateBlockHash())
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&& previousHash.equals(blockchain.get(i)
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.getPreviousHash())
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&& blockchain.get(i)
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.getHash()
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.substring(0, prefix)
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.equals(prefixString);
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if (!flag)
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break;
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}
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assertTrue(flag);
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}
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@AfterClass
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public static void tearDown() {
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blockchain.clear();
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}
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}

‎java-blockchain/src/test/resources/.gitignore‎

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*.class
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#folders#
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/target
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/neoDb*
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/data
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/src/main/webapp/WEB-INF/classes
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*/META-INF/*
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# Packaged files #
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*.jar
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*.war
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*.ear

‎pom.xml‎

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<module>oauth2-framework-impl</module>
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<module>spring-boot-nashorn</module>
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<module>java-blockchain</module>
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</modules>
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</profile>
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<module>spring-security-kerberos</module>
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<module>spring-boot-nashorn</module>
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<module>java-blockchain</module>
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</modules>
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