Java – deliberately cause OutOfMemoryException

Tuesday, August 4th, 2015 

package test;

import java.util.ArrayList;
import java.util.List;

public class OutOfMemory {
    public static void main(String[] args) {
    	final List<String>list = new ArrayList<String>();
    	int i = 1;
    	while(true) {
			list.add(new String(System.nanoTime()+""));
			list.addAll(list);
			System.out.println(i+ " " + Runtime.getRuntime().freeMemory());
			i++;
		}
    	
    }
}

After a few seconds should output something like:


Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
at java.util.Arrays.copyOf(Arrays.java:2245)
at java.util.Arrays.copyOf(Arrays.java:2219)
at java.util.ArrayList.grow(ArrayList.java:213)
at java.util.ArrayList.ensureCapacityInternal(ArrayList.java:187)
at java.util.ArrayList.addAll(ArrayList.java:532)
at test.OutOfMemory.main(OutOfMemory.java:12)

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Oracle XML DB – extracting relational data using XMLTables

Wednesday, April 22nd, 2015

Background

So say you’ve got some XML in an XMLType table. You want to get the data into a “normal” relational table (maybe for faster reporting etc)

The XML looks like this:

<!-- this is one row in the XML table (xml_table) -->
<mxrecords>
   <mxrecord>
       <sysurn></sysurn>
       <description></description>
   </mxrecord>
   <mxrecord>
       <sysurn></sysurn>
       <description></description>
   </mxrecord>
   <mxrecord>
       <sysurn></sysurn>
       <description></description>
   </mxrecord>
</mxrecords>

<!-- this is another row in the XML table -->
<mxrecords>
   <mxrecord>
       <sysurn></sysurn>
       <description></description>
   </mxrecord>
   <mxrecord>
       <sysurn></sysurn>
       <description></description>
   </mxrecord>
   <mxrecord>
       <sysurn></sysurn>
       <description></description>
   </mxrecord>
   <mxrecord>
       <sysurn></sysurn>
       <description></description>
   </mxrecord>
</mxrecords>

The relational table looks like this:

create table reporting(
    sysurn varchar2(50),
    description varchar2(4000)
);

Lets rip the XML into the relational table:

insert into reporting
select mxrecord.* from xml_table xt,
    XMLTABLE(
        '/mxrecords/mxrecord'
        passing xt.OBJECT_VALUE
        columns mxrecord XMLTYPE PATH '/mxrecord') mxrecords,
    XMLTABLE(
        '/mxrecord'
        passing mxrecords.mxrecord
        columns sysurn varchar2(50) PATH '/mxrecord/sysurn',
                description varchar2(4000) PATH '/mxrecord/description') mxrecord

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Oracle XML DB – Inserting XML

Friday, March 20th, 2015

Background

So following on from my previous post about creating XMLType tables in Oracle (either CLOB or schema based), here’s a few quick notes on getting data into the tables.

Inserting Data into XMLType Table

-- Create XMLType Table
CREATE TABLE XML_TABLE OF XMLType 

-- create an Oracle directory to hold the XML (readable by Oracle)
CREATE OR REPLACE DIRECTORY XML as '/home/foo/bar/xml'

-- insert the record (reading from filesystem)
-- filename.xml would be in directory you created above
INSERT INTO XML_TABLE(SELECT XMLTYPE(bfilename('XML', 'filename.xml'), nls_charset_id('UTF8')) FROM dual);

Of course you could easily write some PL/SQL to iterate all files in the directory.

You can also insert XML directly via SQL:

INSERT INTO XML_TABLE VALUES(XMLType('<mxrecords>
        <mxrecord>
            <sysurn>sysurn1</sysurn>
            <eventid>eventid1</eventid>
        </mxrecord>
        <mxrecord>
            <sysurn>sysurn2</sysurn>
            <eventid>eventid2</eventid>
        </mxrecord>
    </mxrecords>'
));

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Oracle XML DB – XMLType table creation

Wednesday, March 18th, 2015

Background

So I’ve been doing some Oracle XML work again. I’d lost my notes from last time (nice touch) so here are my notes from this time:

XMLType Table Creation

CREATE TABLE xml_table OF XMLType

This will create “an xml table” into which you can insert XML. It will check that the XML is well formed before insertion. It will not validate that all documents inserted are the same or that elements contain valid data.

For that we need to create a table based upon an XML Schema/XSD

-- create an Oracle directory where the schema can be stored
create or replace directory XSD as '/home/foo/bar/oracle/xsd'

-- copy your Schema into the directory
-- then
BEGIN
  DBMS_XMLSCHEMA.registerSchema(SCHEMAURL => 'http://www.foobar.com/xsd/my_schema.xsd', SCHEMADOC => bfilename('XSD','my_schema.xsd'));
END;

-- Note: the SCHEMAURL can be basically anything. Oracle uses it as a unique identifier
-- Create the table
CREATE TABLE matter OF xml_table XMLSCHEMA "http://www.foobar.com/xsd/my_schema.xsd" ELEMENT "nameOfTheRootXmlElement";

There are many overloaded versions of DBMS_XMLSCHEMA.registerSchema so you can shortcut some of the steps above. For example:

BEGIN
  DBMS_XMLSCHEMA.registerSchema(
    http://www.foobar.com/xsd/my_schema.xsd',
    bfilename('XSD','my_schema.xsd'),
    TRUE, -- generate required Oracle types. Default = true
    TRUE, -- generate javabeans. Default = false
    FALSE, -- generate errors when registering schema. True = do not
    TRUE -- generate table
);
END;

Full docs for the DBMS_XMLSCHEMA package are here
here

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Java to XML, XML to Java (Marshalling and Unmarshalling)

Wednesday, July 13th, 2011

Introduction

JDK6 and JAXB2.x (which comes with JDK6) make marshalling Java to XML and unmarshalling XML to Java a snap, almost trivial.

Example

Java to XML

package foo.bar;

import java.math.BigDecimal;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.Marshaller;

public class JavaToXML {
	public static void main(String[] args) throws Exception {
		JAXBContext context = JAXBContext.newInstance(Product.class);

		Marshaller m = context.createMarshaller();
		m.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);

		Product object = new Product();
		object.setCode("WI1");
		object.setName("Widget Number One");
		object.setPrice(BigDecimal.valueOf(300.00));

		m.marshal(object, System.out);
	}
}

XML to Java

package foo.bar;

import java.io.File;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBException;
import javax.xml.bind.Unmarshaller;

public class XMLToJava {

	public static void main(String[] args) {
		try {
			JAXBContext jc = JAXBContext.newInstance(Product.class);
			Unmarshaller u = jc.createUnmarshaller();

			File f = new File("product.xml");
			Product product = (Product) u.unmarshal(f);

			System.out.println(product.getCode());
			System.out.println(product.getName());
			System.out.println(product.getPrice());
		} catch (JAXBException e) {
			e.printStackTrace();
		}
	}
}

product.java

Note: the @XmlRootElement is vital here!

package foo.bar;

import java.math.BigDecimal;

import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
public class Product {
	private String code;
	private String name;
	private BigDecimal price;

	public String getCode() {
		return code;
	}
	public void setCode(String code) {
		this.code = code;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public BigDecimal getPrice() {
		return price;
	}
	public void setPrice(BigDecimal price) {
		this.price = price;
	}
}

product.xml

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<product>
    <code>WI1</code>
    <name>Widget Number One</name>
    <price>300.0</price>
</product>

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