Merge pull request #9593 from shapirov103/jdbc-loading

[BAEL-4123] What is the purpose of 'Class.forName(“MY_JDBC_DRIVER”)'?

persistence-modules/core-java-persistence/src/test/java/com/baeldung/jdbc/JdbcDriverLoadingUnitTest.java

@@ -0,0 +1,72 @@
+package com.baeldung.jdbc;
+
+import static org.junit.Assert.*;
+
+import org.junit.Before;
+import org.junit.Test;
+
+import java.sql.Driver;
+import java.sql.DriverManager;
+import java.sql.SQLException;
+
+public class JdbcDriverLoadingUnitTest {
+
+    @Before
+    public void setup() throws SQLException {
+        if (!org.postgresql.Driver.isRegistered()) {
+            org.postgresql.Driver.register();
+        }
+    }
+    /**
+     * Driver is registered automatically with the service provider mechanism.
+     */
+    @Test
+    public void givenJdbcDriverWithSP_whenGettingDriver_thenDriverIsReturned() throws SQLException {
+        Driver driver = getDriver();
+        assertNotNull("Expected initialized postgres driver", driver);
+    }
+
+    /**
+     * Driver is registered automatically, then deregistered.
+     * @throws SQLException
+     */
+    @Test(expected = SQLException.class)
+    public void givenRegisteredJdbcDriver_whenDeregister_thenNoDriverIsReturned() throws SQLException {
+        Driver driver = getDriver();
+        DriverManager.deregisterDriver(driver);
+        driver = getDriver();
+        assertNull(driver);
+    }
+
+    /**
+     * Driver is automatically by the JVM, then deregistered, then registered again.
+     * @throws SQLException
+     * @throws ClassNotFoundException
+     */
+    @Test
+    public void givenNoRegisteredDriver_whenClassForName_thenDriverIsReturned() throws SQLException, ClassNotFoundException {
+        deregisterDriver();
+
+        Driver driver = null;
+        try {
+            driver = getDriver();
+        }
+        catch (SQLException e) {
+            // expected
+        }
+        assertNull("Must fail on attempt to get a deregistered driver", driver);
+
+        org.postgresql.Driver.register(); // This is invoked as part of Class.forName() logic
+
+        driver = getDriver();
+        assertNotNull("Driver must register as part of class loading", driver);
+    }
+
+    private void deregisterDriver() throws SQLException {
+        org.postgresql.Driver.deregister();
+    }
+
+    private Driver getDriver() throws SQLException {
+        return DriverManager.getDriver("jdbc:postgresql:");
+    }
+}