Object Query Language (OQL)

OQL is 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 HAT, OQL adds more flexibility. OQL is based on JavaScript expression language.

OQL query is of the 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. [C is char array name, [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.

OQL Examples

OQL built-in objects, functions

heap object

The heap built-in object supports the following methods: Examples:

functions on individual objects

allocTrace function

This returns allocation site trace of a given Java object if available. allocTrace returns array of frame objects. Each frame object has the following properties:

classof function

Returns Class object of a given Java Object. The result object supports the following properties: Class objects have the following methods: Examples:

forEachReferrer function

calls a callback function for each referrer of a given Java object.

identical function

Returns whether two given Java objects are identical or not.

Example: 

    select identical(heap.findClass("Foo").statics.bar, heap.findClass("AnotherClass").statics.bar)

objectid function

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

reachables function

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:

referrers function

Returns an enumeration of Java objects that hold reference to a given Java object.

Examples:

referees function

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"))

refers function

Returns whether first Java object refers to second Java object or not.

root function

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.

sizeof function

Returns size of the given Java object in bytes Example: 

    select sizeof(o) from [I o

toHtml function

Returns HTML string for the given Java object. Note that this is called automatically for objects selected by select expression. But, it may be useful to print more complex output. Example: print hyperlink in bold font weight 

    select "<b>" + toHtml(o) + "</b>" from java.lang.Object o

Selecting multiple values

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

array/iterator/enumeration manipulation functions

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.

concat function

Concatenates two arrays or enumerations (i.e., returns composite enumeration).

contains function

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.

Example: select all Properties objects that are referred by some static field some class. 

    select p from java.util.Properties p
    where contains(referrers(p), "classof(it).name == 'java.lang.Class'")

count function

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.

Example: print number of classes that have specific name pattern 

    select count(heap.classes(), "/java.io./.test(it.name)")

filter function

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.

Examples:

length function

length function returns number of elements of an array/enumeration.

map function

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)")

max function

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:

Examples:

min function

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:

Examples:

sort function

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:

Examples:

sum function

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

toArray function

This function returns an array that contains elements of the input array/enumeration.

unique function

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'))

More complex examples

Print histogram of each class loader and number of classes loaded by it


   select map(sort(map(heap.objects('java.lang.ClassLoader'), 
   '{ loader: it, count: it.classes.elementCount }'), 'lhs.count < rhs.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.

Show parent-child chain for each class loader instance


   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.

Printing value of all System properties


   select map(filter(heap.findClass('java.lang.System').statics.props.table, 'it != null'), 
            function (it) {
                var res = "";
                while (it != null) {
                    res += it.key.value.toString() + '=' +
                           it.value.value.toString() + '<br>';
                    it = it.next;
                }
                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.