Tuesday, August 4th, 2015
Another interview question.
When this code is executed runnable1 will wait for a lock on “b” and runnable2 will wait for a lock on “a”.
package test;
/**
* Deliberately try and cause deadlock
*
*/
public class DeadLockTest {
public static void main(String[] args) {
final Object a = new Object();
final Object b = new Object();
final Runnable runnable1 = new Runnable() {
public void run() {
synchronized (a) {
try {
Thread.sleep(100); // Adding delay to increase chance of deadlock
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (b) { // We already have "a" abut need "b" too
System.out.println("In runnable1");
}
}
}
};
final Runnable runnable2 = new Runnable() {
public void run() {
synchronized (b) { // Has "b" but needs "a"
synchronized (a) {
System.out.println("In runnable2");
}
}
}
};
new Thread(runnable1).start();
new Thread(runnable2).start();
}
}
Tags: java
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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)
Tags: java
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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
Tags: oracle, xml, xmldb, xmltable, xmltype
Posted in How to's, Oracle | No Comments »
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>'
));
Tags: oracle, xml, xmldb, xmltype
Posted in Examples, How to's, Oracle | No Comments »
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
Tags: oracle, schema, xml, xmldb, xmltype, xsd
Posted in Development, Examples, How to's | No Comments »
