OQL is a SQL-like query language to query Java heap. OQL allows to filter/select information wanted from Java heap. While pre-defined queries such as "show all instances of class X" are already supported by VisualVM, OQL adds more flexibility. OQL is based on JavaScript expression language.
OQL query is of the following form:
select <JavaScript expression to select>
[ from [instanceof] <class name> <identifier>
[ where <JavaScript boolean expression to filter> ] ]
where class name is fully qualified Java class name (example: java.net.URL) or array class name. char[] (or [C) is char array name, java.io.File (or [Ljava.io.File;) is name of java.io.File[] and so on. Note that fully qualified class name does not always uniquely identify a Java class at runtime. There may be more than one Java class with the same name but loaded by different loaders. So, class name is permitted to be id string of the class object. If instanceof keyword is used, subtype objects are selected. If this keyword is not specified, only the instances of exact class specified are selected. Both from and where clauses are optional.
In select and (optional) where clauses, the expression used in JavaScript expression. Java heap objects are wrapped as convenient script objects so that fields may be accessed in natural syntax. For example, Java fields can be accessed with obj.field_name syntax and array elements can be accessed with array[index] syntax. Each Java object selected is bound to a JavaScript variable of the identifier name specified in from clause.
select s from java.lang.String s where s.count >= 100
select a from int[] a where a.length >= 256
select {instance: s, content: s.toString()} from java.lang.String s
where /java/.test(s.toString())
select file.path.toString() from java.io.File file
select classof(cl).name
from instanceof java.lang.ClassLoader cl
select o from instanceof 0xd404b198 o
Note that 0xd404b198 is id of a Class (in a session). This is found by
looking at the id shown in that class's page.
The heap built-in object supports the following methods:
heap.forEachClass(callback);
heap.forEachObject(callback, clazz, includeSubtypes);
clazz is the class whose instances are selected. If not specified, defaults to java.lang.Object. includeSubtypes is a boolean flag
that specifies whether to include subtype instances or not. Default value of
this flag is true.
heap.findClass(className);
where className is name of the class to find. The resulting Class
object has following properties:
Class objects have the following methods:
heap.findObject(stringIdOfObject);
heap.objects(clazz, [includeSubtypes], [filter])
clazz is the class whose instances are selected. If not specified, defaults to java.lang.Object. includeSubtypes is a boolean flag
that specifies whether to include subtype instances or not. Default value of
this flag is true. This method accepts an optional filter expression to filter
the result set of objects.
select heap.livepaths(s) from java.lang.String s
Each element of this array itself is another array. The later array is
contains an objects that are in the 'reference chain' of the path.
select heap.findClass("java.lang.System").statics.props
select heap.findClass("java.lang.System").props
select heap.findClass("java.lang.String").fields.length
select heap.findObject("0xf3800b58")
select filter(heap.classes(), "/java.net./.test(it.name)")
select classof(o).name from instanceof java.lang.ref.Reference o
select heap.findClass("java.io.InputStream").subclasses()
select heap.findClass("java.io.BufferedInputStream").superclasses()
Returns whether two given Java objects are identical or not.
Example:
select identical(heap.findClass("Foo").statics.bar, heap.findClass("AnotherClass").statics.bar)
Returns String id of a given Java object. This id can be passed to heap.findObject and may also be used to compare objects for identity.
Example:
select objectid(o) from java.lang.Object o
Returns an array of Java objects that are transitively referred from the given Java object. Optionally accepts a second parameter that is comma separated field names to be excluded from reachability computation. Fields are written in class_name.field_name pattern.
Examples:
select reachables(p) from java.util.Properties p
select reachables(u, 'java.net.URL.handler') from java.net.URL u
Returns an enumeration of Java objects that hold reference to a given Java object.
Examples:
select count(referrers(o)) from java.lang.Object o
select referrers(f) from java.io.File f
select u from java.net.URL u where count(referrers(u)) > 2
Returns an array of Java objects to which the given Java object directly refers to.
Example: to print all static reference fields of java.io.File class
select referees(heap.findClass("java.io.File"))
Returns whether first Java object refers to second Java object or not.
If given object is a member of root set of objects, this function returns a descriptive Root object describing why it is so. If given object is not a root, then this function returns null.
select sizeof(o) from int[] o
select rsizeof(o) from instanceof java.lang.HashMap o
select "<b>" + toHtml(o) + "</b>" from java.lang.Object o
Multiple values can be selected using JavaScript object literals or arrays.
Example: show name and thread for each thread object
select { name: t.name? t.name.toString() : "null", thread: t }
from instanceof java.lang.Thread t
These functions accept an array/iterator/enumeration and an expression string [or a callback function] as input. These functions iterate the array/iterator/enumeration and apply the expression (or function) on each element. Note that JavaScript objects are associative arrays. So, these functions may also be used with arbitrary JavaScript objects.
Concatenates two arrays or enumerations (i.e., returns composite enumeration).
Returns whether the given array/enumeration contains an element the given boolean expression specified in code. The code evaluated can refer to the following built-in variables.
select p from java.util.Properties p
where contains(referrers(p), "classof(it).name == 'java.lang.Class'")
count function returns the count of elements of the input array/enumeration that satisfy the given boolean expression. The boolean expression code can refer to the following built-in variables.
select count(heap.classes(), "/java.io./.test(it.name)")
filter function returns an array/enumeration that contains elements of the input array/enumeration that satisfy the given boolean expression. The boolean expression code can refer to the following built-in variables.
select filter(heap.classes(), "/java.io./.test(it.name)")
select filter(referrers(u), "! /java.net./.test(classof(it).name)")
from java.net.URL u
length function returns number of elements of an array/enumeration.
Transforms the given array/enumeration by evaluating given code on each element. The code evaluated can refer to the following built-in variables.
map function returns an array/enumeration of values created by repeatedly calling code on each element of input array/enumeration.
Example: show all static fields of java.io.File with name and value
select map(heap.findClass("java.io.File").statics, "index + '=' + toHtml(it)")
returns the maximum element of the given array/enumeration. Optionally accepts code expression to compare elements of the array. By default numerical comparison is used. The comparison expression can use the following built-in variables:
select max(map(heap.objects('java.lang.String', false), 'it.count'))
select max(heap.objects('java.lang.String'), 'lhs.count > rhs.count')
returns the minimum element of the given array/enumeration. Optionally accepts code expression to compare elements of the array. By default numerical comparison is used. The comparison expression can use the following built-in variables:
select min(map(heap.objects('java.util.Vector', false), 'it.elementData.length'))
select min(heap.objects('java.util.Vector'), 'lhs.elementData.length < rhs.elementData.length')
sorts given array/enumeration. Optionally accepts code expression to compare elements of the array. By default numerical comparison is used. The comparison expression can use the following built-in variables:
select sort(heap.objects('char[]'), 'sizeof(lhs) - sizeof(rhs)')
select map(sort(heap.objects('char[]'), 'sizeof(lhs) - sizeof(rhs)'), '{ size: sizeof(it), obj: it }')
returns top N elements of the given array/enumeration. Optionally accepts code expression to compare elements of the array and the number of top elements. By default the first 10 elements in the order of appearence is returned. The comparison expression can use the following built-in variables:
select top(heap.objects('java.lang.String'), 'rhs.count - lhs.count', 5)
select map(top(heap.objects('java.lang.String'), 'rhs.count - lhs.count', 5), '{ length: it.count, obj: it }')
This function returns the sum of all the elements of the given input array or enumeration. Optionally, accepts an expression as second param. This is used to map the input elements before summing those.
Example: return sum of sizes of the reachable objects from each Properties object
select sum(map(reachables(p), 'sizeof(it)'))
from java.util.Properties p
// or omit the map as in ...
select sum(reachables(p), 'sizeof(it)')
from java.util.Properties p
This function returns an array that contains elements of the input array/enumeration.
This function returns an array/enumeration containing unique elements of the given input array/enumeration
Example: select unique char[] instances referenced from Strings. Note that more than one String instance can share the same char[] for the content.
// number of unique char[] instances referenced from any String
select count(unique(map(heap.objects('java.lang.String'), 'it.value')))
// total number of Strings
select count(heap.objects('java.lang.String'))
select map(sort(map(heap.objects('java.lang.ClassLoader'),
'{ loader: it, count: it.classes.elementCount }'), 'rhs.count - lhs.count'),
'toHtml(it) + "<br>"')
The above query uses the fact that, java.lang.ClassLoader has a private field called classes of type java.util.Vector and Vector has a private field named elementCount that is number of elements in the vector. We select multiple values (loader, count) using JavaScript object literal and map function. We sort the result by count (i.e., number of classes loaded) using sort function with comparison expression.
select map(heap.objects('java.lang.ClassLoader'),
function (it) {
var res = '';
while (it != null) {
res += toHtml(it) + "->";
it = it.parent;
}
res += "null";
return res + "<br>";
})
Note that we use parent field of java.lang.ClassLoader class and walk until parent is null using the callback function to map call.
select map(filter(heap.findClass('java.lang.System').props.table, 'it != null && it.key != null && it.value != null'),
function (it) {
var res = it.key.toString() + ' = ' + it.value.toString();
return res;
});
The above query uses the following facts:
Note that this query (and many other queries) may not be stable - because private fields of Java platform classes may be modified/removed without any notification! (implementation detail). But, using such queries on user classes may be safe - given that user has the control over the classes.