警惕 | 警惕，mybatis的size()方法竟然有坑！
Hi ! 我是小小，今天我们又见面了，今日的主要内容是MyBatis的size方法使用的主要的注意事项。
前言
MyBatis 是一个开源的轻量级的半自动化的 ORM 框架，用于面向对象和关系型数据库的映射，其中 xml 文件，和sql语句结合，最大的特点，应用程序sql解耦。OGNL表达式，是MyBatis中的广泛应用，是一种EL语言，用于设置和获取 Java 对象的属性，并且可以对列表进行投影和执行lambda表达式，ognl提供了简单，便于执行的ognl表达式。一个线上服务，经常会出现一个异常，构造各种OGNL表达式为空的情况都会重现该异常，具体的堆栈信息如下：
### Error querying database. Cause: org.apache.ibatis.builder.BuilderException: Error evaluating expression 'list != null and list.size() > 0'. Cause: org.apache.ibatis.ognl.MethodFailedException: Method "size" failed for object [1] [java.lang.IllegalAccessException: Class org.apache.ibatis.ognl.OgnlRuntime can not access a member of class java.util.Collections$SingletonList with modifiers "public"]
### Cause: org.apache.ibatis.builder.BuilderException: Error evaluating expression 'list != null and list.size() > 0'. Cause: org.apache.ibatis.ognl.MethodFailedException: Method "size" failed for object [1] [java.lang.IllegalAccessException: Class org.apache.ibatis.ognl.OgnlRuntime can not access a member of class java.util.Collections$SingletonList with modifiers "public"]
at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:23) org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:107)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:98)
at cn.com.shaobingmm.MybatisBugTest$2.run(MybatisBugTest.java:88)
at java.lang.Thread.run(Thread.java:745)
Caused by: org.apache.ibatis.builder.BuilderException: Error evaluating expression 'list != null and list.size() > 0'. Cause: org.apache.ibatis.ognl.MethodFailedException: Method "size" failed for object [1] [java.lang.IllegalAccessException: Class org.apache.ibatis.ognl.OgnlRuntime can not access a member of class java.util.Collections$SingletonList with modifiers "public"]
at org.apache.ibatis.scripting.xmltags.OgnlCache.getValue(OgnlCache.java
at:47)
at org.apache.ibatis.scripting.xmltags.ExpressionEvaluator.evaluateBoolean(ExpressionEvaluator.java:29)
at org.apache.ibatis.scripting.xmltags.IfSqlNode.apply(IfSqlNode.java:30)
at org.apache.ibatis.scripting.xmltags.MixedSqlNode.apply(MixedSqlNode.java:29)
at org.apache.ibatis.scripting.xmltags.TrimSqlNode.apply(TrimSqlNode.java:51)
at org.apache.ibatis.scripting.xmltags.MixedSqlNode.apply(MixedSqlNode.java:29)
at org.apache.ibatis.scripting.xmltags.DynamicSqlSource.getBoundSql(DynamicSqlSource.java:37)
at org.apache.ibatis.mapping.MappedStatement.getBoundSql(MappedStatement.java:275)
at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:79)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:104)
... 3 more
Caused by: org.apache.ibatis.ognl.MethodFailedException: Method "size" failed for object [1] [java.lang.IllegalAccessException: Class org.apache.ibatis.ognl.OgnlRuntime can not access a member of class java.util.Collections$SingletonList with modifiers "public"]
at org.apache.ibatis.ognl.OgnlRuntime.callAppropriateMethod(OgnlRuntime.java:837)
at org.apache.ibatis.ognl.ObjectMethodAccessor.callMethod(ObjectMethodAccessor.java:61)
at org.apache.ibatis.ognl.OgnlRuntime.callMethod(OgnlRuntime.java:860)
at org.apache.ibatis.ognl.ASTMethod.getValueBody(ASTMethod.java:73)
at org.apache.ibatis.ognl.SimpleNode.evaluateGetValueBody(SimpleNode.java:170)
at org.apache.ibatis.ognl.SimpleNode.getValue(SimpleNode.java:210)
at org.apache.ibatis.ognl.ASTChain.getValueBody(ASTChain.java:109)
at org.apache.ibatis.ognl.SimpleNode.evaluateGetValueBody(SimpleNode.java:170)
at org.apache.ibatis.ognl.SimpleNode.getValue(SimpleNode.java:210)
at org.apache.ibatis.ognl.ASTGreater.getValueBody(ASTGreater.java:49)
at org.apache.ibatis.ognl.SimpleNode.evaluateGetValueBody(SimpleNode.java:170)
at org.apache.ibatis.ognl.SimpleNode.getValue(SimpleNode.java:210)
at org.apache.ibatis.ognl.ASTAnd.getValueBody(ASTAnd.java:56)
at org.apache.ibatis.ognl.SimpleNode.evaluateGetValueBody(SimpleNode.java:170)
at org.apache.ibatis.ognl.SimpleNode.getValue(SimpleNode.java:210)
at org.apache.ibatis.ognl.Ognl.getValue(Ognl.java:333)
at org.apache.ibatis.ognl.Ognl.getValue(Ognl.java:413)
at org.apache.ibatis.ognl.Ognl.getValue(Ognl.java:395)
at org.apache.ibatis.scripting.xmltags.OgnlCache.getValue(OgnlCache.java:45)
... 12 more
List的size方法明明有public，还不可访问，该异常在测试环境未重现，但是在接口的完整调用链路中出错的次数占总的调用次数的0.01%，这是概率性事件。
模拟测试
编写模拟多线程并发读取公司列表的测试代码
多线程下进行压力测试
String resource = "mybatis-config.xml";
InputStream in = null;
try {
in = Resources.getResourceAsStream(resource);
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(in);
final List ids = Collections.singletonList(1L);
final SqlSession session = sqlSessionFactory.openSession();
final CountDownLatch mCountDownLatch = new CountDownLatch(1);
for (int i = 0; i < 50; i++) {
Thread thread = new Thread(new Runnable() {
public void run() {
try {
mCountDownLatch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
for (int k = 0; k < 100; k++) {
session.selectList("CompanyMapper.getCompanysByIds", ids);
}
}
});
thread.start();
}
mCountDownLatch.countDown();
synchronized (MybatisBugTest.class) {
try {
MybatisBugTest.class.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
} catch (IOException e) {
e.printStackTrace();
} catch (Throwable e) {
e.printStackTrace();
} finally {
if (in != null)
try {
in.close();
} catch (IOException e) {
e.printStackTrace();
}
}
上述代码在并发的时候会出现异常。
Caused by: org.apache.ibatis.ognl.MethodFailedException: Method "size" failed for object [1] [java.lang.IllegalAccessException: Class org.apache.ibatis.ognl.OgnlRuntime can not access a member of class java.util.Collections$SingletonList with modifiers "public"]
at org.apache.ibatis.ognl.OgnlRuntime.callAppropriateMethod(OgnlRuntime.java:837)
异常信息表明ognlRuntime类不能访问
查看源码，破案
java.util.Collections的私有成员SingletonList。查看源代码，可以知道锁定在invokeMethod方法上。
public static Object callAppropriateMethod(OgnlContext context, Object source, Object target, String methodName, String propertyName, List methods, Object[] args) throws MethodFailedException {
Object reason = null;
Object[] actualArgs = objectArrayPool.create(args.length);
try {
Method e = getAppropriateMethod(context, source, target, methodName, propertyName, methods, args, actualArgs);
if(e == null || !isMethodAccessible(context, source, e, propertyName)) {
StringBuffer buffer = new StringBuffer();
if(args != null) {
int i = 0;
for(int ilast = args.length - 1; i <= ilast; ++i) {
Object arg = args[i];
buffer.append(arg == null?NULL_STRING:arg.getClass().getName());
if(i < ilast) {
buffer.append(", ");
}
}
}
throw new NoSuchMethodException(methodName + "(" + buffer + ")");
}
Object var14 = invokeMethod(target, e, actualArgs);
return var14;
} catch (NoSuchMethodException var21) {
reason = var21;
} catch (IllegalAccessException var22) {
reason = var22;
} catch (InvocationTargetException var23) {
reason = var23.getTargetException();
} finally {
objectArrayPool.recycle(actualArgs);
}
throw new MethodFailedException(source, methodName, (Throwable)reason);
}
其方法代码
public static Object invokeMethod(Object target, Method method, Object[] argsArray) throws InvocationTargetException, IllegalAccessException {
boolean wasAccessible = true;
if(securityManager != null) {
try {
securityManager.checkPermission(getPermission(method));
} catch (SecurityException var6) {
throw new IllegalAccessException("Method [" + method + "] cannot be accessed.");
}
}
if((!Modifier.isPublic(method.getModifiers()) || !Modifier.isPublic(method.getDeclaringClass().getModifiers())) && !(wasAccessible = method.isAccessible())) {
method.setAccessible(true); （1）
}
Object result = method.invoke(target, argsArray); (3)
if(!wasAccessible) {
method.setAccessible(false); (2)
}
return result;
}
问题出现在meta是一个共享变量，即
public int java.util.Collections$SingletonList.size()
当，第一个线程t1到第一行代码允许method方法可以调用，第二个线程t2，执行到2把方法method设置为不可访问，接着t1又执行，此时行列3会发生异常。
升级版本
lgnl2.7，已经修复了这个问题，所以修复后的代码如下
public static Object invokeMethod(Object target, Method method, Object[] argsArray) throws InvocationTargetException, IllegalAccessException {
boolean syncInvoke = false;
boolean checkPermission = false;
int mHash = method.hashCode();
synchronized(method) {
if(_methodAccessCache.get(Integer.valueOf(mHash)) == null || _methodAccessCache.get(Integer.valueOf(mHash)) == Boolean.TRUE) {
syncInvoke = true;
}
if(_securityManager != null && _methodPermCache.get(Integer.valueOf(mHash)) == null || _methodPermCache.get(Integer.valueOf(mHash)) == Boolean.FALSE) {
checkPermission = true;
}
}
boolean wasAccessible = true;
Object result;
if(syncInvoke) {
synchronized(method) {
if(checkPermission) {
try {
_securityManager.checkPermission(getPermission(method));
_methodPermCache.put(Integer.valueOf(mHash), Boolean.TRUE);
} catch (SecurityException var12) {
_methodPermCache.put(Integer.valueOf(mHash), Boolean.FALSE);
throw new IllegalAccessException("Method [" + method + "] cannot be accessed.");
}
}
if(Modifier.isPublic(method.getModifiers()) && Modifier.isPublic(method.getDeclaringClass().getModifiers())) {
_methodAccessCache.put(Integer.valueOf(mHash), Boolean.FALSE);
} else if(!(wasAccessible = method.isAccessible())) {
method.setAccessible(true);
_methodAccessCache.put(Integer.valueOf(mHash), Boolean.TRUE);
} else {
_methodAccessCache.put(Integer.valueOf(mHash), Boolean.FALSE);
}
result = method.invoke(target, argsArray);
if(!wasAccessible) {
method.setAccessible(false);
}
}
} else {
if(checkPermission) {
try {
_securityManager.checkPermission(getPermission(method));
_methodPermCache.put(Integer.valueOf(mHash), Boolean.TRUE);
} catch (SecurityException var11) {
_methodPermCache.put(Integer.valueOf(mHash), Boolean.FALSE);
throw new IllegalAccessException("Method [" + method + "] cannot be accessed.");
}
}
result = method.invoke(target, argsArray);
}
return result;
}
关于作者
我是小小，一个生于二线，活在一线城市的程序猿，我是小小，我们下期再见。
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