BAEL-1818 - A Guide to Connection Pools in Java (#4735)

* Strange git issue with README.MD, wouldn't revert the file

* Initial Commit

* Initial Commit

* Update pom.xml

* Update pom.xml

* Initial Commit

* Update pom.xml

* Update source files

* Update source files

* Update source files

* Update source files

* Update Application.java

* Update pom.xml

* Update HikariCPDataSourceUnitTest class

* Update HikariCPDataSourceUnitTest.java

* Update pom.xml

* Update unit test classes

* Update BasicConnectionPoolUnitTest.java

* Fix indentation in DBCDDataSource class

* Update DBCPDataSource.java

* Update BasicConnectionPool class

* Update BasicConnectionPool class

* Update BasicConnectionPool.java

* Update BasicConnectionPool.java

* Update pom.xml

* Update pom.xml

* BAEL-1818 Refactored getConnection(), added shutdown(), cleaned up pom.xml

* BAEL-1818 Removed getConnectionPool(), upgraded c3po version

* BAEL-1818 Deleted obsolete connectionpool module

* BAEL-1818 Deleted obsolete connectionpool module

core-java/pom.xml

@@ -1,5 +1,5 @@
 <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">
+         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.baeldung</groupId>
     <artifactId>core-java</artifactId>
@@ -73,9 +73,10 @@
         <dependency>
             <groupId>org.assertj</groupId>
             <artifactId>assertj-core</artifactId>
-            <version>${assertj.version}</version>
+            <version>${assertj-core.version}</version>
             <scope>test</scope>
         </dependency>
+
         <dependency>
             <groupId>commons-codec</groupId>
             <artifactId>commons-codec</artifactId>
@@ -146,6 +147,21 @@
             <artifactId>icu4j</artifactId>
             <version>${icu4j.version}</version>
         </dependency>
+        <dependency>
+            <groupId>org.apache.commons</groupId>
+            <artifactId>commons-dbcp2</artifactId>
+            <version>${commons-dbcp2.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>com.zaxxer</groupId>
+            <artifactId>HikariCP</artifactId>
+            <version>${HikariCP.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>com.mchange</groupId>
+            <artifactId>c3p0</artifactId>
+            <version>${c3p0.version}</version>
+        </dependency>
     </dependencies>
 
     <build>
@@ -301,7 +317,7 @@
                     </arguments>
                 </configuration>
             </plugin>
-
+            
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-javadoc-plugin</artifactId>
@@ -348,6 +364,7 @@
                     <plugin>
                         <groupId>org.codehaus.mojo</groupId>
                         <artifactId>exec-maven-plugin</artifactId>
+                        <version>${exec-maven-plugin.version}</version>
                         <executions>
                             <execution>
                                 <id>run-benchmarks</id>
@@ -375,6 +392,7 @@
     </profiles>
 
     <properties>
+        
         <!-- marshalling -->
         <jackson.version>2.8.5</jackson.version>
         <gson.version>2.8.2</gson.version>
@@ -395,16 +413,23 @@
         <vavr.version>0.9.0</vavr.version>
 
         <!-- testing -->
-        <assertj.version>3.6.1</assertj.version>
+        <assertj-core.version>3.10.0</assertj-core.version>
 
+        <!-- JDBC pooling frameworks -->
+        <commons-dbcp2.version>2.4.0</commons-dbcp2.version>
+        <HikariCP.version>3.2.0</HikariCP.version>
+        <c3p0.version>0.9.5.2</c3p0.version>
+        
         <!-- maven plugins -->
         <maven-surefire-plugin.version>2.19.1</maven-surefire-plugin.version>
         <springframework.spring-web.version>4.3.4.RELEASE</springframework.spring-web.version>
         <springframework.boot.spring-boot-starter.version>1.5.8.RELEASE</springframework.boot.spring-boot-starter.version>
+        
         <javamoney.moneta.version>1.1</javamoney.moneta.version>
         <h2database.version>1.4.197</h2database.version>
         <esapi.version>2.1.0.1</esapi.version>
         <jmh-core.version>1.19</jmh-core.version>
+       
         <jmh-generator-annprocess.version>1.19</jmh-generator-annprocess.version>
         <maven-javadoc-plugin.version>3.0.0-M1</maven-javadoc-plugin.version>
         <javax.mail.version>1.5.0-b01</javax.mail.version>
@@ -412,7 +437,8 @@
         <onejar-maven-plugin.version>1.4.4</onejar-maven-plugin.version>
         <maven-shade-plugin.version>3.1.1</maven-shade-plugin.version>
         <spring-boot-maven-plugin.version>2.0.3.RELEASE</spring-boot-maven-plugin.version>
+        <exec-maven-plugin.version>1.6.0</exec-maven-plugin.version>
         <icu4j.version>61.1</icu4j.version>
     </properties>
 
-</project>
+</project>

core-java/src/main/java/com/baeldung/connectionpool/connectionpools/BasicConnectionPool.java

@@ -0,0 +1,87 @@
+package com.baeldung.connectionpool.connectionpools;
+
+import java.sql.Connection;
+import java.sql.DriverManager;
+import java.sql.SQLException;
+import java.util.ArrayList;
+import java.util.List;
+
+public class BasicConnectionPool implements ConnectionPool {
+
+    private final String url;
+    private final String user;
+    private final String password;
+    private final List<Connection> connectionPool;
+    private final List<Connection> usedConnections = new ArrayList<>();
+    private static final int INITIAL_POOL_SIZE = 10;
+    private final int MAX_POOL_SIZE = 20;
+    
+    public static BasicConnectionPool create(String url, String user, String password) throws SQLException {
+        List<Connection> pool = new ArrayList<>(INITIAL_POOL_SIZE);
+        for (int i = 0; i < INITIAL_POOL_SIZE; i++) {
+            pool.add(createConnection(url, user, password));
+        }
+        return new BasicConnectionPool(url, user, password, pool);
+    }
+    
+    private BasicConnectionPool(String url, String user, String password, List<Connection> connectionPool) {
+        this.url = url;
+        this.user = user;
+        this.password = password;
+        this.connectionPool = connectionPool;
+    }
+    
+    @Override
+    public Connection getConnection() throws SQLException {
+        if (connectionPool.size() == 0) {
+            if (usedConnections.size() < MAX_POOL_SIZE) {
+                connectionPool.add(createConnection(url, user, password));
+            } else {
+                throw new RuntimeException("Maximum pool size reached, no available connections!");
+            }
+        }
+
+        Connection connection = connectionPool.remove(connectionPool.size() - 1);
+        usedConnections.add(connection);
+        return connection;
+    }
+    
+    @Override
+    public boolean releaseConnection(Connection connection) {
+        connectionPool.add(connection);
+        return usedConnections.remove(connection);
+    }
+    
+    private static Connection createConnection(String url, String user, String password) throws SQLException {
+        return DriverManager.getConnection(url, user, password);
+    }
+    
+    public int getSize() {
+        return connectionPool.size() + usedConnections.size();
+    }
+
+    @Override
+    public String getUrl() {
+        return url;
+    }
+
+    @Override
+    public String getUser() {
+        return user;
+    }
+
+    @Override
+    public String getPassword() {
+        return password;
+    }
+
+    public void shutdown() throws SQLException {
+        for (Connection c : usedConnections) {
+            this.releaseConnection(c);
+        }
+        for (Connection c : connectionPool) {
+            c.close();
+        }
+        connectionPool.clear();
+    }
+}

core-java/src/main/java/com/baeldung/connectionpool/connectionpools/C3poDataSource.java

@@ -0,0 +1,28 @@
+package com.baeldung.connectionpool.connectionpools;
+
+import com.mchange.v2.c3p0.ComboPooledDataSource;
+import java.beans.PropertyVetoException;
+import java.sql.Connection;
+import java.sql.SQLException;
+
+public class C3poDataSource {
+
+    private static final ComboPooledDataSource cpds = new ComboPooledDataSource();
+
+    static {
+        try {
+            cpds.setDriverClass("org.h2.Driver");
+            cpds.setJdbcUrl("jdbc:h2:mem:test");
+            cpds.setUser("user");
+            cpds.setPassword("password");
+        } catch (PropertyVetoException e) {
+            e.printStackTrace();
+        }
+    }
+    
+    public static Connection getConnection() throws SQLException {
+        return cpds.getConnection();
+    }
+    
+    private C3poDataSource(){}
+}

core-java/src/main/java/com/baeldung/connectionpool/connectionpools/ConnectionPool.java

@@ -0,0 +1,18 @@
+package com.baeldung.connectionpool.connectionpools;
+
+import java.sql.Connection;
+import java.sql.SQLException;
+import java.util.List;
+
+public interface ConnectionPool {
+
+    Connection getConnection() throws SQLException;
+    
+    boolean releaseConnection(Connection connection);
+    
+    String getUrl();
+    
+    String getUser();
+
+    String getPassword();
+}   

core-java/src/main/java/com/baeldung/connectionpool/connectionpools/DBCPDataSource.java

@@ -0,0 +1,25 @@
+package com.baeldung.connectionpool.connectionpools;
+
+import java.sql.Connection;
+import java.sql.SQLException;
+import org.apache.commons.dbcp2.BasicDataSource;
+
+public class DBCPDataSource {
+    
+    private static final BasicDataSource ds = new BasicDataSource();
+    
+    static {
+        ds.setUrl("jdbc:h2:mem:test");
+        ds.setUsername("user");
+        ds.setPassword("password");
+        ds.setMinIdle(5);
+        ds.setMaxIdle(10);
+        ds.setMaxOpenPreparedStatements(100);
+    }
+    
+    public static Connection getConnection() throws SQLException {
+        return ds.getConnection();
+    }
+    
+    private DBCPDataSource(){}
+}

core-java/src/main/java/com/baeldung/connectionpool/connectionpools/HikariCPDataSource.java

@@ -0,0 +1,28 @@
+package com.baeldung.connectionpool.connectionpools;
+
+import com.zaxxer.hikari.HikariConfig;
+import com.zaxxer.hikari.HikariDataSource;
+import java.sql.Connection;
+import java.sql.SQLException;
+
+public class HikariCPDataSource {
+    
+    private static final HikariConfig config = new HikariConfig();
+    private static final HikariDataSource ds;
+    
+    static {
+        config.setJdbcUrl("jdbc:h2:mem:test");
+        config.setUsername("user");
+        config.setPassword("password");
+        config.addDataSourceProperty("cachePrepStmts", "true");
+        config.addDataSourceProperty("prepStmtCacheSize", "250");
+        config.addDataSourceProperty("prepStmtCacheSqlLimit", "2048");
+        ds = new HikariDataSource(config);
+    }
+    
+    public static Connection getConnection() throws SQLException {
+        return ds.getConnection();
+    }
+    
+    private HikariCPDataSource(){}
+}

core-java/src/test/java/com/baeldung/connectionpool/BasicConnectionPoolUnitTest.java

@@ -0,0 +1,69 @@
+package com.baeldung.connectionpool;
+
+import com.baeldung.connectionpool.connectionpools.BasicConnectionPool;
+import com.baeldung.connectionpool.connectionpools.ConnectionPool;
+import java.sql.Connection;
+import java.sql.SQLException;
+import java.util.List;
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.fail;
+
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+public class BasicConnectionPoolUnitTest {
+    
+    private static ConnectionPool connectionPool;
+
+    @BeforeClass
+    public static void setUpBasicConnectionPoolInstance() throws SQLException {
+        connectionPool = BasicConnectionPool.create("jdbc:h2:mem:test", "user", "password");
+    }
+    
+    @Test
+    public void givenBasicConnectionPoolInstance_whenCalledgetConnection_thenCorrect() throws Exception {
+        assertTrue(connectionPool.getConnection().isValid(1));
+    }
+    
+    @Test
+    public void givenBasicConnectionPoolInstance_whenCalledreleaseConnection_thenCorrect() throws Exception {
+        Connection connection = connectionPool.getConnection();
+        assertThat(connectionPool.releaseConnection(connection)).isTrue();
+    }
+    
+    @Test
+    public void givenBasicConnectionPoolInstance_whenCalledgetUrl_thenCorrect() {
+        assertThat(connectionPool.getUrl()).isEqualTo("jdbc:h2:mem:test");
+    }
+    
+    @Test
+    public void givenBasicConnectionPoolInstance_whenCalledgetUser_thenCorrect() {
+        assertThat(connectionPool.getUser()).isEqualTo("user");
+    }
+    
+    @Test
+    public void givenBasicConnectionPoolInstance_whenCalledgetPassword_thenCorrect() {
+        assertThat(connectionPool.getPassword()).isEqualTo("password");
+    }
+
+    @Test(expected = RuntimeException.class)
+    public void givenBasicConnectionPoolInstance_whenAskedForMoreThanMax_thenError() throws Exception {
+        // this test needs to be independent so it doesn't share the same connection pool as other tests
+        ConnectionPool cp = BasicConnectionPool.create("jdbc:h2:mem:test", "user", "password");
+        final int MAX_POOL_SIZE = 20;
+        for (int i = 0; i < MAX_POOL_SIZE + 1; i++) {
+            cp.getConnection();
+        }
+        fail();
+    }
+
+    @Test
+    public void givenBasicConnectionPoolInstance_whenSutdown_thenEmpty() throws Exception {
+        ConnectionPool cp = BasicConnectionPool.create("jdbc:h2:mem:test", "user", "password");
+        assertThat(((BasicConnectionPool)cp).getSize()).isEqualTo(10);
+
+        ((BasicConnectionPool) cp).shutdown();
+        assertThat(((BasicConnectionPool)cp).getSize()).isEqualTo(0);
+    }
+}

core-java/src/test/java/com/baeldung/connectionpool/C3poDataSourceUnitTest.java

@@ -0,0 +1,14 @@
+package com.baeldung.connectionpool;
+
+import com.baeldung.connectionpool.connectionpools.C3poDataSource;
+import java.sql.SQLException;
+import static org.junit.Assert.assertTrue;
+import org.junit.Test;
+
+public class C3poDataSourceUnitTest {
+    
+    @Test
+    public void givenC3poDataSourceClass_whenCalledgetConnection_thenCorrect() throws SQLException {
+        assertTrue(C3poDataSource.getConnection().isValid(1));
+    }   
+}

core-java/src/test/java/com/baeldung/connectionpool/DBCPDataSourceUnitTest.java

@@ -0,0 +1,14 @@
+package com.baeldung.connectionpool;
+
+import com.baeldung.connectionpool.connectionpools.DBCPDataSource;
+import java.sql.SQLException;
+import static org.junit.Assert.assertTrue;
+import org.junit.Test;
+
+public class DBCPDataSourceUnitTest {
+    
+    @Test
+    public void givenDBCPDataSourceClass_whenCalledgetConnection_thenCorrect() throws SQLException {
+        assertTrue(DBCPDataSource.getConnection().isValid(1));
+    }   
+}

core-java/src/test/java/com/baeldung/connectionpool/HikariCPDataSourceUnitTest.java

@@ -0,0 +1,14 @@
+package com.baeldung.connectionpool;
+
+import com.baeldung.connectionpool.connectionpools.HikariCPDataSource;
+import java.sql.SQLException;
+import static org.junit.Assert.assertTrue;
+import org.junit.Test;
+
+public class HikariCPDataSourceUnitTest {
+    
+    @Test
+    public void givenHikariDataSourceClass_whenCalledgetConnection_thenCorrect() throws SQLException {
+        assertTrue(HikariCPDataSource.getConnection().isValid(1));
+    }   
+}