Deprecate JdbcMySqlDialect.INSTANCE and MySqlDialect.MYSQL_IDENTIFIER_PROCESSING.

MySql does not have a fixed Identifierprocessing, therefore predefined instances doesn't make sense.

Closes #2060
See https://dev.mysql.com/doc/refman/8.4/en/identifier-case-sensitivity.html

spring-data-jdbc/src/main/java/org/springframework/data/jdbc/core/dialect/JdbcMySqlDialect.java

@@ -42,7 +42,17 @@ import org.springframework.lang.NonNull;
  */
 public class JdbcMySqlDialect extends MySqlDialect implements JdbcDialect {
 
-	public static final JdbcMySqlDialect INSTANCE = new JdbcMySqlDialect();
+	/**
+	 * Predefined instance of the {@literal JdbcMySqlDialect}.
+	 *
+	 * @deprecated Use the constructor instead. There is no one correct MySqlDialect, since the behaviour of MySql depends
+	 *             on various configuration options. See
+	 * 
+	 *             <pre>
+	 * <a href="https://dev.mysql.com/doc/refman/8.4/en/identifier-case-sensitivity.html">Identifier Case Sensitivity</a>
+	 *             </pre>
+	 */
+	@Deprecated(forRemoval = true, since = "4.0") public static final JdbcMySqlDialect INSTANCE = new JdbcMySqlDialect();
 
 	public JdbcMySqlDialect(IdentifierProcessing identifierProcessing) {
 		super(identifierProcessing);

spring-data-jdbc/src/test/java/org/springframework/data/jdbc/core/convert/IdGeneratingEntityCallbackTest.java

@@ -35,6 +35,7 @@ import org.springframework.data.relational.core.conversion.MutableAggregateChang
 import org.springframework.data.relational.core.mapping.RelationalMappingContext;
 import org.springframework.data.relational.core.mapping.Sequence;
 import org.springframework.data.relational.core.mapping.Table;
+import org.springframework.data.relational.core.sql.IdentifierProcessing;
 import org.springframework.jdbc.core.RowMapper;
 import org.springframework.jdbc.core.namedparam.NamedParameterJdbcOperations;
 import org.springframework.jdbc.core.namedparam.SqlParameterSource;
@@ -50,6 +51,8 @@ class IdGeneratingEntityCallbackTest {
 
 	@Mock NamedParameterJdbcOperations operations;
 	RelationalMappingContext relationalMappingContext;
+	private JdbcMySqlDialect mySqlDialect = new JdbcMySqlDialect(
+			IdentifierProcessing.create(new IdentifierProcessing.Quoting("`"), IdentifierProcessing.LetterCasing.LOWER_CASE));
 
 	@BeforeEach
 	void setUp() {
@@ -64,8 +67,8 @@ class IdGeneratingEntityCallbackTest {
 
 		NamedParameterJdbcOperations operations = mock(NamedParameterJdbcOperations.class);
 
-		IdGeneratingEntityCallback subject = new IdGeneratingEntityCallback(relationalMappingContext,
-				JdbcMySqlDialect.INSTANCE, operations);
+		IdGeneratingEntityCallback subject = new IdGeneratingEntityCallback(relationalMappingContext, mySqlDialect,
+				operations);
 
 		EntityWithSequence processed = (EntityWithSequence) subject.onBeforeSave(new EntityWithSequence(),
 				MutableAggregateChange.forSave(new EntityWithSequence()));
@@ -76,8 +79,8 @@ class IdGeneratingEntityCallbackTest {
 	@Test // GH-1923
 	void entityIsNotMarkedWithTargetSequence() {
 
-		IdGeneratingEntityCallback subject = new IdGeneratingEntityCallback(relationalMappingContext,
-				JdbcMySqlDialect.INSTANCE, operations);
+		IdGeneratingEntityCallback subject = new IdGeneratingEntityCallback(relationalMappingContext, mySqlDialect,
+				operations);
 
 		NoSequenceEntity processed = (NoSequenceEntity) subject.onBeforeSave(new NoSequenceEntity(),
 				MutableAggregateChange.forSave(new NoSequenceEntity()));

spring-data-relational/src/main/java/org/springframework/data/relational/core/dialect/MySqlDialect.java

@@ -37,9 +37,18 @@ public class MySqlDialect extends AbstractDialect {
 
 	/**
 	 * MySQL defaults for {@link IdentifierProcessing}.
+	 * 
+	 * @deprecated Construct your own {@link IdentifierProcessing}. There is no one standard identifier processing for
+	 *             MySql.See
+	 * 
+	 *             <pre>
+	 * 	  <a href=
+	"https://dev.mysql.com/doc/refman/8.4/en/identifier-case-sensitivity.html">Identifier Case Sensitivity</a>
+	 *             </pre>
 	 */
-	public static final IdentifierProcessing MYSQL_IDENTIFIER_PROCESSING = IdentifierProcessing.create(new Quoting("`"),
-			LetterCasing.LOWER_CASE);
+	@Deprecated(forRemoval = true,
+			since = "4.0") public static final IdentifierProcessing MYSQL_IDENTIFIER_PROCESSING = IdentifierProcessing
+					.create(new Quoting("`"), LetterCasing.LOWER_CASE);
 
 	/**
 	 * Singleton instance.
@@ -47,8 +56,7 @@ public class MySqlDialect extends AbstractDialect {
 	 * @deprecated use either the {@code org.springframework.data.r2dbc.dialect.MySqlDialect} or
 	 *             {@code org.springframework.data.jdbc.core.dialect.JdbcMySqlDialect}
 	 */
-	@Deprecated(forRemoval = true)
-	public static final MySqlDialect INSTANCE = new MySqlDialect();
+	@Deprecated(forRemoval = true) public static final MySqlDialect INSTANCE = new MySqlDialect();
 
 	private final IdentifierProcessing identifierProcessing;