Object Query Language(OQL)

定义

Object Query Language 即对象查询语言，用于在JVM dump映象文件中通过一种类SQL的查询语言查找指定的对象。

软件

• Sun/Oracle JDK软件自带 jhat
• Memory Analyzer (MAT) http://www.eclipse.org/mat/

语法

  select <JavaScript expression to select>
    [ from [instanceof] <class name> <identifier>
    [ where <JavaScript boolean expression to filter> ] ]

<class name>	完整的类名（如果用接口名即使加了instanceof也似乎查不了）
instanceof	查找指定类包含其子类的实例
<identifier>	相关于数据库的别名，在查找结果和条件中使用

例子

// 查找所有长度超过100的字符串
select s from java.lang.String s where s.count >= 100

// 查找所有长度超过256的int数组
select a from [I a where a.length >= 256

// 查找所有匹配指定正则表达式的字符串
select s.value.toString() from java.lang.String s where /java/((s && s.value) ? s.value.toString(): "")

// 查找所有文件的path
select file.path.value.toString() from java.io.File file

// 查找所有ClassLoader的名字
select classof(cl).name from instanceof java.lang.ClassLoader cl

// 查询所有value不同的整数
select unique(x, "it.value") from java.lang.Integer x

内建对象、函数

• heap对象

heap对象的内建函数：

• heap.forEachClass -- 为每个Java类调用一次callback回调函数

  heap.forEachClass(callback);

• heap.forEachObject -- 为每个Java对象调用callback回调函数

  heap.forEachObject(callback, clazz, includeSubtypes);

• heap.findClass -- finds Java Class of given name

  heap.findClass(className);

• heap.findObject -- finds object from given object id

  heap.findObject(stringIdOfObject);

• heap.classes -- returns an enumeration of all Java classes
• heap.objects -- returns an enumeration of Java objects

  heap.objects(clazz, [includeSubtypes], [filter])

• heap.finalizables -- returns an enumeration of Java objects that are pending to be finalized.
• heap.livepaths -- return an array of paths by which a given object is alive. This method accepts optional second parameter that is a boolean flag. This flag tells whether to include paths with weak reference(s) or not. By default, paths with weak reference(s) are not included.

  select heap.livepaths(s) from java.lang.String s

• heap.roots -- returns an Enumeration of Roots of the heap. Each Root object has the following properties:
• id - String id of the object that is referred by this root
• type - descriptive type of Root (JNI Global, JNI Local, Java Static etc)
• description - String description of the Root
• referrer - Thread Object or Class object that is responsible for this root or null

• functions on individual objects
• allocTrace(jobject)
• classof(jobject)
• forEachReferrer(callback, jobject)
• identical(o1, o2)
• objectid(jobject)
• reachables(jobject, excludedFields)
• referrers(jobject)
• referees(jobject)
• refers(jobject)
• root(jobject)
• sizeof(jobject)
• toHtml(obj)
• array/iterator/enumeration manipulation functions
• concat(array1/enumeration1, array2/enumeration2)
• contains(array/enumeration, expression)
• count(array/enumeration, expression)
• filter(array/enumeration, expression)
• length(array/enumeration)
• map(array/enumeration, expression)
• max(array/enumeration, [expression])
• min(array/enumeration, [expression])
• sort(array/enumeration, [expression])
• sum(array/enumeration, [expression])
• toArray(array/enumeration)
• unique(array/enumeration, [expression])

MAT OQL Object Query Language

参考资料

[1]. jhat/oqlhelp/
[2]. http://visualvm.java.net/oqlhelp.html