Object Query Language (OQL)

This section describes what is OQL (Object Query Language). OQL statement syntax and examples are also provided.

Warning: "jhat" has been discontinued since JDK 9. But if you still have JDK 1.8 installed, you can continue to use it to browser heap dump files generated by JDK 9 and JDK 10.

What Is OQL (Object Query Language)? 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 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:

select all Strings of length 100 or more:
    select s from java.lang.String s where s.count >= 100

select all int arrays of length 256 or more:
    select a from [I a where a.length >= 256

show content of Strings that match a regular expression:
    select s.value.toString() from java.lang.String s 
    where /java/(s.value.toString())

show path value of all File objects:
    select file.path.value.toString() from java.io.File file

show names of all ClassLoader classes:
    select classof(cl).name 
    from instanceof java.lang.ClassLoader cl

show instances of the Class identified by given id string:
    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.

See next section on how to run OQL statements on the "jhat" Web server.

Last update: 2018.

Table of Contents

 About This Book

 Java Tools Terminology

 Java Tools Included in JDK

 javac - The Java Program Compiler

 jar - The JAR File Tool

 jlink - The JRE Linker

 jmod - The JMOD File Tool

 jimage - The JIMAGE File Tool

 java - The Java Program Launcher

 jdb - The Java Debugger

 jcmd - The JVM Diagnostic Tool

 jconsole - Java Monitoring and Management Console

 jstat - JVM Statistics Monitoring Tool

JVM Troubleshooting Tools

 JVM Troubleshooting Tools in JDK

 jinfo - VM Option Value Checker

 jinfo - Changing HotSpot VM Option

 jstack - Stack Tracer to Generate Thread Dump

 What Is a Thread Dump?

 Java Thread Deadlock Demo Program

 jstack - Detecting Java Thread Deadlocks

 jmap - JVM Heap Dump Tool

 Printing Histogram of Java Object Heap

 jmap - Generating Heap Dump File

 jhat - Java Heap Analysis Tool

 jhat - Starting Web Server on a Heap Dump File

 Listing Instance Counts of All Classes

 Browsing Object Instance Values

Object Query Language (OQL)

 Searching for Instances with OQL Statements

 jvisualvm (Java VisualVM) - JVM Visual Tool

 jmc - Java Mission Control

 javap - The Java Class File Disassembler

 keytool - Public Key Certificate Tool

 jarsigner - JAR File Signer

 jshell - Java Language Shell

 native2ascii - Native-to-ASCII Encoding Converter

 Outdated Tutorials

 References

 Full Version in PDF/EPUB