从 Java 对象类型到 SQL 类型的标准映射在哪里定义？

Question

在setObject的 JDBC 文档中，我发现了这句话：

JDBC 规范指定了从 Java 对象类型到 SQL 类型的标准映射。

但是，我找不到此映射的定义位置。 我发现的只有 这个。 但这仅指定了 SQL 类型和 JDBC 类型之间的映射。 但我没有看到，我是如何从 JDBC 类型转换为 Java 类型的。

更具体地说，我想知道这个代码是否保证永远不会抛出InvalidCastException ：

int type = resultSet.getMetadata().getColumnType(1);
if (type == Types.LONGNVARCHAR || type == Types.CHAR) {
    String abc = (String)resultSet.getObject(1);
}

在这种情况下，人们会期望String是非常直观的，但是对于许多其他 SQL 类型，我没有这样的直觉。

我只是找不到一张表告诉我可以根据java.sql.Types常量的知识安全地执行哪些强制转换。

Answer 1

您可以在此处下载 JDBC 的官方规范（因为您链接到 Java 8 文档，所以该规范适用于 JDBC 版本 4.2）。 具体在附录的表 B-3 中。

对于 JDBC 类型CHAR和LONGNVARCHAR ，Java 类型确实是String ，因此您的强制转换是安全的。

Answer 2

您可以在此处找到类型映射：

映射 SQL 和 Java 类型(Java 1.5)

您要查找的部分在表 8.9.3 映射到 Java 对象类型的 JDBC 类型中：

 JDBC Type Java Object Type ============= ============================== CHAR String VARCHAR String LONGVARCHAR String NUMERIC java.math.BigDecimal DECIMAL java.math.BigDecimal BIT Boolean TINYINT Integer SMALLINT Integer INTEGER Integer BIGINT Long REAL Float FLOAT Double DOUBLE Double BINARY byte[] VARBINARY byte[] LONGVARBINARY byte[] DATE java.sql.Date TIME java.sql.Time TIMESTAMP java.sql.Timestamp DISTINCT Object type of underlying type CLOB Clob BLOB Blob ARRAY Array STRUCT Struct or SQLData REF Ref JAVA_OBJECT underlying Java class

Answer 3

Mybatis Generator 源码地址如下：

https://github.com/mybatis/generator/blob/fe8232ad26214e06d349f697ffb67ec09036a842/core/mybatis-generator-core/src/main/java/org/mybatis/generator/internal/types/JavaTypeResolverDefaultImpl.java

这是源代码的副本（防止URL无效）

/*
 *    Copyright 2006-2020 the original author or authors.
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *       http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 */
package org.mybatis.generator.internal.types;

import java.math.BigDecimal;
import java.sql.Types;
import java.util.Date;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Properties;

import org.mybatis.generator.api.IntrospectedColumn;
import org.mybatis.generator.api.JavaTypeResolver;
import org.mybatis.generator.api.dom.java.FullyQualifiedJavaType;
import org.mybatis.generator.config.Context;
import org.mybatis.generator.config.PropertyRegistry;
import org.mybatis.generator.internal.util.StringUtility;

public class JavaTypeResolverDefaultImpl implements JavaTypeResolver {

    protected List<String> warnings;

    protected final Properties properties;

    protected Context context;

    protected boolean forceBigDecimals;
    protected boolean useJSR310Types;

    protected final Map<Integer, JdbcTypeInformation> typeMap;

    public JavaTypeResolverDefaultImpl() {
        super();
        properties = new Properties();
        typeMap = new HashMap<>();

        typeMap.put(Types.ARRAY, new JdbcTypeInformation("ARRAY", //$NON-NLS-1$
                new FullyQualifiedJavaType(Object.class.getName())));
        typeMap.put(Types.BIGINT, new JdbcTypeInformation("BIGINT", //$NON-NLS-1$
                new FullyQualifiedJavaType(Long.class.getName())));
        typeMap.put(Types.BINARY, new JdbcTypeInformation("BINARY", //$NON-NLS-1$
                new FullyQualifiedJavaType("byte[]"))); //$NON-NLS-1$
        typeMap.put(Types.BIT, new JdbcTypeInformation("BIT", //$NON-NLS-1$
                new FullyQualifiedJavaType(Boolean.class.getName())));
        typeMap.put(Types.BLOB, new JdbcTypeInformation("BLOB", //$NON-NLS-1$
                new FullyQualifiedJavaType("byte[]"))); //$NON-NLS-1$
        typeMap.put(Types.BOOLEAN, new JdbcTypeInformation("BOOLEAN", //$NON-NLS-1$
                new FullyQualifiedJavaType(Boolean.class.getName())));
        typeMap.put(Types.CHAR, new JdbcTypeInformation("CHAR", //$NON-NLS-1$
                new FullyQualifiedJavaType(String.class.getName())));
        typeMap.put(Types.CLOB, new JdbcTypeInformation("CLOB", //$NON-NLS-1$
                new FullyQualifiedJavaType(String.class.getName())));
        typeMap.put(Types.DATALINK, new JdbcTypeInformation("DATALINK", //$NON-NLS-1$
                new FullyQualifiedJavaType(Object.class.getName())));
        typeMap.put(Types.DATE, new JdbcTypeInformation("DATE", //$NON-NLS-1$
                new FullyQualifiedJavaType(Date.class.getName())));
        typeMap.put(Types.DECIMAL, new JdbcTypeInformation("DECIMAL", //$NON-NLS-1$
                new FullyQualifiedJavaType(BigDecimal.class.getName())));
        typeMap.put(Types.DISTINCT, new JdbcTypeInformation("DISTINCT", //$NON-NLS-1$
                new FullyQualifiedJavaType(Object.class.getName())));
        typeMap.put(Types.DOUBLE, new JdbcTypeInformation("DOUBLE", //$NON-NLS-1$
                new FullyQualifiedJavaType(Double.class.getName())));
        typeMap.put(Types.FLOAT, new JdbcTypeInformation("FLOAT", //$NON-NLS-1$
                new FullyQualifiedJavaType(Double.class.getName())));
        typeMap.put(Types.INTEGER, new JdbcTypeInformation("INTEGER", //$NON-NLS-1$
                new FullyQualifiedJavaType(Integer.class.getName())));
        typeMap.put(Types.JAVA_OBJECT, new JdbcTypeInformation("JAVA_OBJECT", //$NON-NLS-1$
                new FullyQualifiedJavaType(Object.class.getName())));
        typeMap.put(Types.LONGNVARCHAR, new JdbcTypeInformation("LONGNVARCHAR", //$NON-NLS-1$
                new FullyQualifiedJavaType(String.class.getName())));
        typeMap.put(Types.LONGVARBINARY, new JdbcTypeInformation(
                "LONGVARBINARY", //$NON-NLS-1$
                new FullyQualifiedJavaType("byte[]"))); //$NON-NLS-1$
        typeMap.put(Types.LONGVARCHAR, new JdbcTypeInformation("LONGVARCHAR", //$NON-NLS-1$
                new FullyQualifiedJavaType(String.class.getName())));
        typeMap.put(Types.NCHAR, new JdbcTypeInformation("NCHAR", //$NON-NLS-1$
                new FullyQualifiedJavaType(String.class.getName())));
        typeMap.put(Types.NCLOB, new JdbcTypeInformation("NCLOB", //$NON-NLS-1$
                new FullyQualifiedJavaType(String.class.getName())));
        typeMap.put(Types.NVARCHAR, new JdbcTypeInformation("NVARCHAR", //$NON-NLS-1$
                new FullyQualifiedJavaType(String.class.getName())));
        typeMap.put(Types.NULL, new JdbcTypeInformation("NULL", //$NON-NLS-1$
                new FullyQualifiedJavaType(Object.class.getName())));
        typeMap.put(Types.NUMERIC, new JdbcTypeInformation("NUMERIC", //$NON-NLS-1$
                new FullyQualifiedJavaType(BigDecimal.class.getName())));
        typeMap.put(Types.OTHER, new JdbcTypeInformation("OTHER", //$NON-NLS-1$
                new FullyQualifiedJavaType(Object.class.getName())));
        typeMap.put(Types.REAL, new JdbcTypeInformation("REAL", //$NON-NLS-1$
                new FullyQualifiedJavaType(Float.class.getName())));
        typeMap.put(Types.REF, new JdbcTypeInformation("REF", //$NON-NLS-1$
                new FullyQualifiedJavaType(Object.class.getName())));
        typeMap.put(Types.SMALLINT, new JdbcTypeInformation("SMALLINT", //$NON-NLS-1$
                new FullyQualifiedJavaType(Short.class.getName())));
        typeMap.put(Types.STRUCT, new JdbcTypeInformation("STRUCT", //$NON-NLS-1$
                new FullyQualifiedJavaType(Object.class.getName())));
        typeMap.put(Types.TIME, new JdbcTypeInformation("TIME", //$NON-NLS-1$
                new FullyQualifiedJavaType(Date.class.getName())));
        typeMap.put(Types.TIMESTAMP, new JdbcTypeInformation("TIMESTAMP", //$NON-NLS-1$
                new FullyQualifiedJavaType(Date.class.getName())));
        typeMap.put(Types.TINYINT, new JdbcTypeInformation("TINYINT", //$NON-NLS-1$
                new FullyQualifiedJavaType(Byte.class.getName())));
        typeMap.put(Types.VARBINARY, new JdbcTypeInformation("VARBINARY", //$NON-NLS-1$
                new FullyQualifiedJavaType("byte[]"))); //$NON-NLS-1$
        typeMap.put(Types.VARCHAR, new JdbcTypeInformation("VARCHAR", //$NON-NLS-1$
                new FullyQualifiedJavaType(String.class.getName())));
        // JDK 1.8 types
        typeMap.put(Types.TIME_WITH_TIMEZONE, new JdbcTypeInformation("TIME_WITH_TIMEZONE", //$NON-NLS-1$
                new FullyQualifiedJavaType("java.time.OffsetTime"))); //$NON-NLS-1$
        typeMap.put(Types.TIMESTAMP_WITH_TIMEZONE, new JdbcTypeInformation("TIMESTAMP_WITH_TIMEZONE", //$NON-NLS-1$
                new FullyQualifiedJavaType("java.time.OffsetDateTime"))); //$NON-NLS-1$
    }

    @Override
    public void addConfigurationProperties(Properties properties) {
        this.properties.putAll(properties);
        forceBigDecimals = StringUtility
                .isTrue(properties
                        .getProperty(PropertyRegistry.TYPE_RESOLVER_FORCE_BIG_DECIMALS));
        useJSR310Types = StringUtility
                .isTrue(properties
                        .getProperty(PropertyRegistry.TYPE_RESOLVER_USE_JSR310_TYPES));
    }

    @Override
    public FullyQualifiedJavaType calculateJavaType(
            IntrospectedColumn introspectedColumn) {
        FullyQualifiedJavaType answer = null;
        JdbcTypeInformation jdbcTypeInformation = typeMap
                .get(introspectedColumn.getJdbcType());

        if (jdbcTypeInformation != null) {
            answer = jdbcTypeInformation.getFullyQualifiedJavaType();
            answer = overrideDefaultType(introspectedColumn, answer);
        }

        return answer;
    }

    protected FullyQualifiedJavaType overrideDefaultType(IntrospectedColumn column,
            FullyQualifiedJavaType defaultType) {
        FullyQualifiedJavaType answer = defaultType;

        switch (column.getJdbcType()) {
        case Types.BIT:
            answer = calculateBitReplacement(column, defaultType);
            break;
        case Types.DATE:
            answer = calculateDateType(column, defaultType);
            break;
        case Types.DECIMAL:
        case Types.NUMERIC:
            answer = calculateBigDecimalReplacement(column, defaultType);
            break;
        case Types.TIME:
            answer = calculateTimeType(column, defaultType);
            break;
        case Types.TIMESTAMP:
            answer = calculateTimestampType(column, defaultType);
            break;
        default:
            break;
        }

        return answer;
    }

    protected FullyQualifiedJavaType calculateDateType(IntrospectedColumn column, FullyQualifiedJavaType defaultType) {
        FullyQualifiedJavaType answer;

        if (useJSR310Types) {
            answer = new FullyQualifiedJavaType("java.time.LocalDate"); //$NON-NLS-1$
        } else {
            answer = defaultType;
        }

        return answer;
    }

    protected FullyQualifiedJavaType calculateTimeType(IntrospectedColumn column, FullyQualifiedJavaType defaultType) {
        FullyQualifiedJavaType answer;

        if (useJSR310Types) {
            answer = new FullyQualifiedJavaType("java.time.LocalTime"); //$NON-NLS-1$
        } else {
            answer = defaultType;
        }

        return answer;
    }

    protected FullyQualifiedJavaType calculateTimestampType(IntrospectedColumn column,
            FullyQualifiedJavaType defaultType) {
        FullyQualifiedJavaType answer;

        if (useJSR310Types) {
            answer = new FullyQualifiedJavaType("java.time.LocalDateTime"); //$NON-NLS-1$
        } else {
            answer = defaultType;
        }

        return answer;
    }

    protected FullyQualifiedJavaType calculateBitReplacement(IntrospectedColumn column,
            FullyQualifiedJavaType defaultType) {
        FullyQualifiedJavaType answer;

        if (column.getLength() > 1) {
            answer = new FullyQualifiedJavaType("byte[]"); //$NON-NLS-1$
        } else {
            answer = defaultType;
        }

        return answer;
    }

    protected FullyQualifiedJavaType calculateBigDecimalReplacement(IntrospectedColumn column,
            FullyQualifiedJavaType defaultType) {
        FullyQualifiedJavaType answer;

        if (column.getScale() > 0 || column.getLength() > 18 || forceBigDecimals) {
            answer = defaultType;
        } else if (column.getLength() > 9) {
            answer = new FullyQualifiedJavaType(Long.class.getName());
        } else if (column.getLength() > 4) {
            answer = new FullyQualifiedJavaType(Integer.class.getName());
        } else {
            answer = new FullyQualifiedJavaType(Short.class.getName());
        }

        return answer;
    }

    @Override
    public String calculateJdbcTypeName(IntrospectedColumn introspectedColumn) {
        String answer = null;
        JdbcTypeInformation jdbcTypeInformation = typeMap
                .get(introspectedColumn.getJdbcType());

        if (jdbcTypeInformation != null) {
            answer = jdbcTypeInformation.getJdbcTypeName();
        }

        return answer;
    }

    @Override
    public void setWarnings(List<String> warnings) {
        this.warnings = warnings;
    }

    @Override
    public void setContext(Context context) {
        this.context = context;
    }

    public static class JdbcTypeInformation {
        private final String jdbcTypeName;

        private final FullyQualifiedJavaType fullyQualifiedJavaType;

        public JdbcTypeInformation(String jdbcTypeName,
                FullyQualifiedJavaType fullyQualifiedJavaType) {
            this.jdbcTypeName = jdbcTypeName;
            this.fullyQualifiedJavaType = fullyQualifiedJavaType;
        }

        public String getJdbcTypeName() {
            return jdbcTypeName;
        }

        public FullyQualifiedJavaType getFullyQualifiedJavaType() {
            return fullyQualifiedJavaType;
        }
    }
}

希望这可以帮到你 ：）