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入门示例

Java代码

package foobar; 

import org.flywaydb.core.Flyway;

public class App {
public static void main(String[] args) {
Flyway flyway = new Flyway();
// 指定数据源
flyway.setDataSource("jdbc:mysql://localhost/test", "root", "root");
// 开始数据迁移
flyway.migrate();
}
}

在classpath下添加SQL文件 db/migration/V1__Create_person_table.sql

create table PERSON ( 
    ID int not null, 
    NAME varchar(100) not null 
); 

运行程序,在test数据库会自动创建PERSON表。

后续新增表和字段,只需在db/migration目录下新增SQL文件,格式为V${version}__${name}.sql,version值依次增加,比如V2__name.sql,V3__name.sql。

原理介绍

Flyway的数据库迁移的实现原理是,从classpath或文件系统中找到符合规则的数据库迁移脚本,比如db/migration目录下命名规则为V${version}__${name}.sql的文件,将脚本按照version进行排序,依次执行。执行过的脚本会作为一条记录,存储在schema_version表中。当下次执行迁移时,判断脚本已经执行,则跳过。

MigrationResolver接口负责查找数据库迁移脚本,方法为resolveMigrations(),数据库迁移脚本用ResolvedMigration对象表示。MigrationResolver包含多种实现类,比如SqlMigrationResolver会从classpath下查找sql文件。查询通过Scanner类实现,Location类指定查询路径,sql文件的命名规则需要符合V${version}__${name}.sql,规则中的前缀V、后缀.sql、分隔符__均在FlywayConfiguration接口中定义。另一种实现类JdbcMigrationResolver会从classpath下查找实现JdbcMigration接口的类,类的命名规则需要符合V${version}__${name}。需要扩展自己的实现类,可以继承BaseMigrationResolver。

MetaDataTable接口负责查找已执行的数据库迁移脚本,方法为findAppliedMigrations(),已执行的数据库迁移脚本用AppliedMigration对象表示。MetaDataTable只有一种实现类MetaDataTableImpl,从数据库schema_version表查询所有记录。

ResolvedMigration集合包含了已经执行的AppliedMigration集合,在执行ResolvedMigration前,需要对比AppliedMigration,找到已执行和未执行的ResolvedMigration,对比通过MigrationInfoServiceImpl.refresh()实现。已执行的ResolvedMigration需要校验文件有没有发生变化,有变更则提示错误。未执行的ResolvedMigration依次执行,执行结果记录在schema_version表中。

Flyway的主要方法:

public class Flyway { 
   /**数据库迁移*/ 
   public int migrate(); 
   /**校验已执行的迁移操作的变更情况*/ 
   public void validate(); 
   /**清理数据库*/ 
   public void clean(); 
   /**设置数据库的基准版本*/ 
   public void baseline(); 
   /**删除执行错误的迁移记录*/ 
   public void repair(); 
   /**准备执行环境,并执行Command操作,以上方法都调用了execute()来执行操作*/ 
   <T> T execute(Command<T> command); 
} 

接下来我们分析Flyway.migrate()代码执行的主逻辑。

public void migrate() { 
  //由Flyway.execute()准备Command.execute()执行所需要的参数 
  return execute(new Command<Integer>() { 
    public Integer execute(Connection connectionMetaDataTable, 
      MigrationResolver migrationResolver,  MetaDataTable metaDataTable,  
      DbSupport dbSupport, Schema[] schemas, FlywayCallback[] flywayCallbacks) { 
      //为了简化代码,忽略参数传递 
      doMigrate(); 
} 

});
}

private void doMigrate() {
//校验已执行的迁移操作的变更情况
if (validateOnMigrate) {
doValidate(connectionMetaDataTable, dbSupport, migrationResolver,
metaDataTable, schemas, flywayCallbacks, true);
}

//如果尚未进行数据迁移,即schema_version表中不存在数据,
//并且数据库不为空,则插入一条baseline信息
if (!metaDataTable.exists()) {
//数据库不为空
if (!nonEmptySchemas.isEmpty()) {
//插入一条baseline信息
new DbBaseline(connectionMetaDataTable, dbSupport, metaDataTable,
schemas[0], baselineVersion, baselineDescription, flywayCallbacks).baseline();
}
}

//进行数据迁移
DbMigrate dbMigrate = new DbMigrate(connectionUserObjects, dbSupport,
metaDataTable,schemas[0], migrationResolver, ignoreFailedFutureMigration,
Flyway.this);
return dbMigrate.migrate();
}

接下来看DbMigrate.migrate()的代码片段。

MigrationInfoServiceImpl对比ResolvedMigration和AppliedMigration对象,找出需要执行数据库迁移脚本,通过pending()方法返回。最后执行数据库迁移脚本。

public int migrate() { 
  int migrationSuccessCount = 0; 
  while (true) { 
    int count = metaDataTable.lock(new Callable<Integer>() { 
  <span class="hljs-meta">@Override</span> 
  <span class="hljs-function"><span class="hljs-keyword">public</span> Integer <span class="hljs-title">call</span><span class="hljs-params">()</span> </span>{ 
    <span class="hljs-comment">//为了简化代码,忽略参数传递</span> 
    <span class="hljs-keyword">return</span> doMigrate(); 
  } 
} 
<span class="hljs-keyword">if</span> (count == <span class="hljs-number">0</span>) { 
  <span class="hljs-comment">// No further migrations available</span> 
  <span class="hljs-keyword">break</span>; 
} 
migrationSuccessCount += count; 

}
return migrationSuccessCount;
}

private int doMigrate() {
//收集已经入库的数据库迁移记录,和以文件形式存在的数据库迁移脚本
MigrationInfoServiceImpl infoService = new MigrationInfoServiceImpl(
migrationResolver, metaDataTable, configuration.getTarget(),
configuration.isOutOfOrder(), true, true, true);
infoService.refresh();

//infoService.pending()记录将要执行的数据库迁移脚本
LinkedHashMap<MigrationInfoImpl, Boolean> group =
groupMigrationInfo(infoService.pending());
if (!group.isEmpty()) {
//执行数据库迁移操作
applyMigrations(group);
}
}

DbMigrate.doMigrateGroup() 执行数据库迁移脚本

private void doMigrateGroup() { 
  //执行迁移脚本 
  migration.getResolvedMigration().getExecutor().execute(connectionUserObjects); 

//存入数据库
AppliedMigration appliedMigration = new AppliedMigration(migration.getVersion(),
migration.getDescription(), migration.getType(), migration.getScript(),
migration.getResolvedMigration().getChecksum(), executionTime, true);
metaDataTable.addAppliedMigration(appliedMigration);
}

扩展练习

在一个已经上线的游戏项目中引入Flyway框架,对于已经存在的游戏服,只执行新增的sql语句,对于新搭建的游戏服,需要创建数据库,并执行新增的sql语句。

在这个需求中,需要做的是对于新搭建的游戏服,需要创建数据库。我们可以通过FlywayCallback实现这一点,如果指定数据库为空,则执行初始化数据库的语句。

代码如下:

public static void main(String[] args) { 
    final Flyway flyway = new Flyway(); 
    flyway.setDataSource("jdbc:mysql://localhost/test", "root", "root"); 
FlywayCallback flywayCallback = <span class="hljs-keyword">new</span> BaseFlywayCallback() { 
     
    <span class="hljs-meta">@Override</span> 
    <span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">beforeMigrate</span><span class="hljs-params">(Connection connection)</span> </span>{ 
        DbSupport dbSupport = DbSupportFactory.createDbSupport(connection, <span class="hljs-keyword">false</span>); 
        <span class="hljs-keyword">if</span>(!hasTable(dbSupport)) { 
            initDb(dbSupport); 
        } 
    } 
 
    <span class="hljs-function"><span class="hljs-keyword">private</span> <span class="hljs-keyword">boolean</span> <span class="hljs-title">hasTable</span><span class="hljs-params">(DbSupport dbSupport)</span> </span>{ 
        Schema&lt;?&gt; schema = dbSupport.getOriginalSchema(); 
        Table[] tables = schema.allTables(); 
        <span class="hljs-keyword">if</span>(tables == <span class="hljs-keyword">null</span> || tables.length == <span class="hljs-number">0</span>) { 
            <span class="hljs-keyword">return</span> <span class="hljs-keyword">false</span>; 
        } 
        <span class="hljs-comment">//忽略表 schema_version</span> 
        <span class="hljs-keyword">if</span>(tables.length == <span class="hljs-number">1</span> &amp;&amp;  
            tables[<span class="hljs-number">0</span>].getName().equalsIgnoreCase(<span class="hljs-string">"schema_version"</span>)) { 
            <span class="hljs-keyword">return</span> <span class="hljs-keyword">false</span>; 
        } 
        <span class="hljs-keyword">return</span> <span class="hljs-keyword">true</span>; 
    } 
     
 
    <span class="hljs-function"><span class="hljs-keyword">private</span>  <span class="hljs-keyword">void</span> <span class="hljs-title">initDb</span><span class="hljs-params">(DbSupport dbSupport)</span> </span>{ 
        Scanner scanner = <span class="hljs-keyword">new</span> Scanner( 
          Thread.currentThread().getContextClassLoader()); 
        Resource[] resources = scanner.scanForResources( 
          <span class="hljs-keyword">new</span> Location(<span class="hljs-string">"db/init"</span>), <span class="hljs-string">""</span>, <span class="hljs-string">".sql"</span>); 
        <span class="hljs-keyword">if</span>(resources == <span class="hljs-keyword">null</span> || resources.length == <span class="hljs-number">0</span>) { 
            <span class="hljs-keyword">throw</span> <span class="hljs-keyword">new</span> RuntimeException( 
              <span class="hljs-string">"db/init/*.sql not found in the classpath. "</span>); 
        } 
        <span class="hljs-keyword">for</span>(Resource resource : resources) { 
          SqlMigrationExecutor executor = <span class="hljs-keyword">new</span> SqlMigrationExecutor( 
              dbSupport, resource, PlaceholderReplacer.NO_PLACEHOLDERS, flyway); 
          executor.execute(dbSupport.getJdbcTemplate().getConnection()); 
        } 
    } 
}; 
 
List&lt;FlywayCallback&gt; callbacks = <span class="hljs-keyword">new</span> ArrayList&lt;FlywayCallback&gt;( 
  Arrays.asList(flyway.getCallbacks())); 
callbacks.add(flywayCallback); 
flyway.setCallbacks(callbacks.toArray(<span class="hljs-keyword">new</span> FlywayCallback[callbacks.size()])); 
 
flyway.setBaselineOnMigrate(<span class="hljs-keyword">true</span>); 
flyway.repair(); 
flyway.migrate(); 

}

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