Searching for Instances with OQL Statements

This section provides a tutorial example on how to write an OQL (Object Query Language) query statement to search for object instances with specific conditions.

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.

To test the power of OQL, I want to find those Object[] instances created by my program, excluding Object[] created by the JVM platform.

1. Run a Web browser with http://localhost:7000. The heap dump first page shows up.

2. Click the link of "Execute Object Query Language (OQL) query". The OQL query page shows up.

3. Enter the following query and click the Execute button:

select i from [Ljava.lang.Object; i where i.length == 10

Cool. "jhat" returns a list of instances that matches my query condition:


jhat - Object Query Language (OQL)
jhat - Object Query Language (OQL)

But the previous OQL query is not good enough to return only Object[] instances I wanted. The query below will do a better job. Try it.

select i from [Ljava.lang.Object; i where i.length == 10
   && i[5] != null && classof(i[5]).name == '[Ljava.lang.String;'

Watch out the OQL statement syntax. It takes JavaScript expressions only, not SQL expressions.

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