Java Tools Tutorials - Herong's Tutorial Examples - v6.24, by Herong Yang
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 to JDK 12.
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.
Table of Contents
javac - The Java Program Compiler
java - The Java Program Launcher
jpackage - Binary Package Builder
javadoc - The Java Document Generator
jdeps - The Java Class Dependency Analyzer
jdeprscan - The Java Deprecated API Scanner
jcmd - The JVM Diagnostic Tool
jconsole - Java Monitoring and Management Console
jstat - JVM Statistics Monitoring Tool
JVM Troubleshooting Tools in JDK
jinfo - VM Option Value Checker
jinfo - Changing HotSpot VM Option
jstack - Stack Tracer to Generate Thread Dump
Java Thread Deadlock Demo Program
jstack - Detecting Java Thread Deadlocks
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
Searching for Instances with OQL Statements
jhsdb - The Java HotSpot Debugger
jvisualvm (Java VisualVM) - JVM Visual Tool
javap - The Java Class File Disassembler
keytool - Public Key Certificate Tool
jrunscript - Script Code Shell
native2ascii - Native-to-ASCII Encoding Converter