Understanding the first level JPA cache
I can bet that every Java developer at least heard about L1 (aka EntityManager or Session) cache. But is your level of understanding it good enough? If you're not sure, consider going through this post.
At first, we need to know what the persistence context is. Following EntityManager JavaDoc we know, that:
"A persistence context is a set of entity instances in which for any persistent entity identity there is a unique entity instance. Within the persistence context, the entity instances and their lifecycle are managed."
In fact, the first level cache is the same as the persistence context. That means operations such as persist(), merge(), remove() are changing only internal collections in the context and are not synchronized to the underlying database. What is the mosts important here is what happens when you invoke the clear() method. It clears the L1 cache. But we know L1 == persistence context. Does it mean clearing L1 removes all entities? In fact yes - all entities will be dropped and never synchronized to the database. That's not a secret, it states in the documentation - "Unflushed changes made to the entity (...) will not be synchronized to the database." But who cares about the docs? :)
So how does it look in practice? Take a look at the code below:
If you omit flush() the entity won't hit the database. It will live only in your code, and after leaving method which created this object will be lost. Let's take a look at the next sample:
What will be the value of the name property after finishing this code? Of course still "old name", because in the moment when setName() has been invoked the entity is no longer managed and it has to be merged with the persistent context (by invoking em.merge(myEntity) method) to be the subject of the dirty checking.
But I'm not calling flush() method in my code and everything works!? But do you call clear()? That's what I thought. What is the default flush behavior? JPA by default flushes changes on commit as well as every query execution (FlushModeType.AUTO). If you change it to COMMIT (with em.setFlushMode(FlushModeType.COMMIT) method) then flush will occur (as name suggests) only on commit.
Deep understanding of the L1 behavior is especially important when dealing with batch operations. Why? If you want such operation to be effective, we must manually flush changes from time to time (let's say every 100 operations). Do you know, that flush() doesn't clear the persistence context? So what? Flush is not cheap because it must process all entities in the context to see if there is anything to synchronize with the database. If you won't clear the context manually immediately after flush(), every next operation will take longer and longer. To be honest this time grows exponentially, which seems to be sufficient reason to remember about mentioned technique.
If you're interested in deeper dive into persistence context please feel free to clone and play with this repository, which contains all described cases and examples.
At first, we need to know what the persistence context is. Following EntityManager JavaDoc we know, that:
"A persistence context is a set of entity instances in which for any persistent entity identity there is a unique entity instance. Within the persistence context, the entity instances and their lifecycle are managed."
In fact, the first level cache is the same as the persistence context. That means operations such as persist(), merge(), remove() are changing only internal collections in the context and are not synchronized to the underlying database. What is the mosts important here is what happens when you invoke the clear() method. It clears the L1 cache. But we know L1 == persistence context. Does it mean clearing L1 removes all entities? In fact yes - all entities will be dropped and never synchronized to the database. That's not a secret, it states in the documentation - "Unflushed changes made to the entity (...) will not be synchronized to the database." But who cares about the docs? :)
So how does it look in practice? Take a look at the code below:
em.persist(myEntity); // saves entity to the context em.flush(); // triggers insert into database em.clear(); // removes entity from the context == entity is no longer managed
If you omit flush() the entity won't hit the database. It will live only in your code, and after leaving method which created this object will be lost. Let's take a look at the next sample:
myEntity.setName("old name"); em.persist(myEntity); em.flush(); em.clear(); myEntity.setName("new name"); em.flush();
What will be the value of the name property after finishing this code? Of course still "old name", because in the moment when setName() has been invoked the entity is no longer managed and it has to be merged with the persistent context (by invoking em.merge(myEntity) method) to be the subject of the dirty checking.
But I'm not calling flush() method in my code and everything works!? But do you call clear()? That's what I thought. What is the default flush behavior? JPA by default flushes changes on commit as well as every query execution (FlushModeType.AUTO). If you change it to COMMIT (with em.setFlushMode(FlushModeType.COMMIT) method) then flush will occur (as name suggests) only on commit.
Deep understanding of the L1 behavior is especially important when dealing with batch operations. Why? If you want such operation to be effective, we must manually flush changes from time to time (let's say every 100 operations). Do you know, that flush() doesn't clear the persistence context? So what? Flush is not cheap because it must process all entities in the context to see if there is anything to synchronize with the database. If you won't clear the context manually immediately after flush(), every next operation will take longer and longer. To be honest this time grows exponentially, which seems to be sufficient reason to remember about mentioned technique.
If you're interested in deeper dive into persistence context please feel free to clone and play with this repository, which contains all described cases and examples.
Comments
very useful. I don’t know what I would have done if I had not
encountered such a step like this.
Digital Marketing Course In Chennai
Digital Marketing Training In Chennai
Seo Training in Chennai
Digital Marketing Course in Chennai
Digital Marketing Training in Chennai
Online Digital Marketing Training
SEO Training in Chennai
Abinitio Online Training Instittue
Microsoft Azure Online Training Instittue
App V Online Training Instittue
Digital Marketing Training
linux training in hyderabad
Angularjs Training in Chennai
Angularjs course in Chennai
Big Data Training in Chennai
German Classes in Chennai
AngularJS Training in Porur
AngularJS Training in Velachery
AngularJS Training in Adyar
Oracle Training in Chennai
Oracle Training institute in chennai
Unix Training in Chennai
Unix Shell Scripting Training in Chennai
LINUX Training in Chennai
LINUX Course in Chennai
Oracle Training in Velachery
Oracle Training in Tambaram
PHP Training in Chennai
web designing course in chennai
JAVA Training in Chennai
Hadoop Training in Chennai
Selenium Training in Chennai
German Classes in chennai
web designing course in chennai
web designing course in Porur
Aviation Academy in Chennai
Air hostess training in Chennai
Airport management courses in Chennai
Ground staff training in Chennai
Aviation Courses in Chennai
air hostess course in Chennai
Airport Management Training in Chennai
airport ground staff training courses in Chennai
All those who are into e-commerce and wants to know more about it and re-commerce.
Mainframe Training in Hyderabad
Well said! Nice content and thanks for sharing the post. Great written and useful info, Please keep blogging.
Pega Training in Chennai
Pega Course
Primavera Training in Chennai
Excel Training in Chennai
Corporate Training in Chennai
Embedded System Course Chennai
Tableau Training in Chennai
Spark Training in Chennai
Thanks & Regards,
VRIT Professionals,
No.1 Leading Web Designing Training Institute In Chennai.
And also those who are looking for
Web Designing Training Institute in Chennai
SEO Training Institute in Chennai
Photoshop Training Institute in Chennai
PHP & Mysql Training Institute in Chennai
Android Training Institute in Chennai
web designing course with placement
php training with placement in chennai
magento developer training | magento course in chennai
Corporate TRaining Spring Framework
Project Centers in Chennai For CSE
Spring Training in Chennai
Web Designing Training Institute in Chennai | web design training center in chennai | web designing course in chennai with placement
Mobile Application Development Courses in chennai
Data Science Training in Chennai | Data Science courses in Chennai
Professional packers and movers in chennai | PDY Packers | Household Goods Shifting
Web Designing Training Institute in Chennai | Web Designing courses in Chennai
PPC Companies in Delhi
SEO Company in Delhi
SEO Company in Delhi NCR
SMO Company in Delhi
SMO Company in India
SEO Training in Chennai
SEO Training in Bangalore
SEO Training in Coimbatore
SEO Training in Madurai
SEO Course in Chennai
SEO Course in Chennai
SEO Course in Bangalore
SEO Course in Coimbatore
This post gives a piece of excellent information. From this blog i learned lot of useful information from this blog
DOT NET Training in Chennai
DOT NET Training in Bangalore
DOT NET Training Institutes in Bangalore
DOT NET Course in Bangalore
Best DOT NET Training Institutes in Bangalore
DOT NET Institute in Bangalore
Dot NET Training in Marathahalli
AWS Training in Bangalore
Data Science Courses in Bangalore
DevOps Training in Bangalore
custom application development services
Software development company
software application development company
offshore software development company
custom software development company
Ai & Artificial Intelligence Course in Chennai
PHP Training in Chennai
Ethical Hacking Course in Chennai Blue Prism Training in Chennai
UiPath Training in Chennai
online store developer in USA |Web Developer in USA | E-Store Developer in USA | Website Designer in California |Website Developer in California |Portal Developer in California |
online store developer in USA |Web Developer in USA | E-Store Developer in USA | Website Designer in California |Website Developer in California |Portal Developer in California |
Vermicompost manufacturers in Tamilnadu | Vermicompost in Tamilnadu
Vermicompost Manufacturers | Vermicompost Suppliers
Vermicompost in Coimbatore | Vermicompost manufacturers in Chennai
Vermicompost in chennai | Best Vermicompost in chennai
big data projects for cse final year students
Java Training in Chennai
Final Year Projects for CSE
Java Training in Chennai
AI Training in Hyderabad
list to string python
what is data structure in python
what is polymorphism in oops with example
numpy example in python
python interview questions and answers pdf
types of data structure in python
vé máy bay đi Mỹ giá bao nhiêu
vé máy bay tết
đặt vé máy bay đi toronto canada
ve may bay di Phap gia re
vé máy bay từ việt nam sang anh
mua vé máy bay giá rẻ ở đâu
combo du lịch đà nẵng hội an
combo nha trang 3 ngày 2 đêm
visa trung quốc online
cách ly khách sạn trọn gói
Best Air hostess Training Institute in Chennai
Best Ground staff Training institute in Chennai
Best Ground staff Academy in Chennai
Best Travel Agency in Madurai | Travels in Madurai
Madurai Travels | Best Travels in Madurai
Tours and Travels in Madurai | Best Tour Operators in Madurai
applications of python
full stack developer salary
need of python programming
hadoop career path
java interview questions and answers
selenium interview questions and answers
HIV/AIDS Complete cure test result in Tamilnadu | Hiv aids treatment in bangalore
Best hiv doctor in india | Hiv positive treatment in india
Medicine for aids in india | HBSag complete cure for Herbal in india
Python Training In Bangalore | Python Online Training
Artificial Intelligence Training In Bangalore | Artificial Intelligence Online Training
Data Science Training In Bangalore | Data Science Online Training
Machine Learning Training In Bangalore | Machine Learning Online Training
AWS Training In Bangalore | AWS Online Training
IoT Training In Bangalore | IoT Online Training
Adobe Experience Manager (AEM) Training In Bangalore | Adobe Experience Manager (AEM) Online Training
Oracle Apex Training In Bangalore | Oracle Apex Online Training
vé máy bay đi seoul
mua vé máy bay đi nhật bản
vé máy bay giá rẻ từ việt nam đi nhật
giá vé máy bay đi đài loan
vé máy bay sài gòn đài bắc
vietjet bay cao hùng
UiPath Training In Bangalore | UiPath Online Training
AS400 Training In Bangalore | AS400 Online Training
Docker and Kubernetes Training In Bangalore | Docker and Kubernetes Online Training
Go Language Training In Bangalore | Go Language Online Training
Oracle Apps Technical Training In Bangalore | Oracle Apps Technical Online Training
UI UX Design Training In Bangalore | UI UX Design Online Training
Pyspark Training In Bangalore | Pyspark Online Training
React Native and Redux Training In Bangalore | API Online Training
Adobe Sitecatalyst Training In Bangalore | Adobe Sitecatalyst Online Training
Python with Django Training In Bangalore | Python with Django Online Training
JAVA Course in Chennai
Selenium Course in Chennai
Python course in Chennai
AWS Course in Chennai
Data Science Training in Chennai
DevOps certification in Chennai
Vermicompost Manufacturers | Vermicompost Suppliers
Vermicompost in chennai | Vermicompost manufacturers in Chennai
Best Machine Learning Classroom Training in Bangalore
UI UX Design Agency
UI UX Design Firms
Python Classes in Chennai
Python Classes Near Me
Best Python Training in Bangalore
SEO Training in Chennai
SEO Course Online
SEO Training in Bangalore
SEO Training in Coimbatore
escort service in gurgaon
escort service in Noida
Escorts Service in Mahipalpur.
vé máy bay từ úc về việt nam bao nhiêu
dat ve may bay tu han quoc ve viet nam
dat ve may bay tu han quoc ve viet nam
mua vé máy bay từ mỹ về việt nam hãng eva
Giá vé máy bay đi Nhật Bản Vietnam Airline
vé máy bay từ los angeles về việt nam
Different Types of Software Testing
Different types of Software Testing Tools
call girl in agra escort service in agra
Escorts Service In Mathura
Meerut Escorts
Noida Escorts Service
Varanasi Escort Service
Allahabad Call Girls
ETL tools
Python
Python Classes in Chennai
learn python online
Python Classes in Coimbatore
What is google flutter?
Benefits of Flutter
TOEFL Coaching in Chennai
TOEFL Classes in Chennai
Basics of Protocol Testing
What is Protocol Testing?
Hibernate Training in Chennai
Spring Hibernate Training in Chennai
Software Testing Course in Bangalore
Best Software Testing Institute in Bangalore
How to face interview for freshers
How to face interview for fresher
Python Training in Hyderabad
Python Classes in Gurgaon
react web development
video interview platform
video interview software
Great blog with good information.
Protocol Training Online
Protocol Testing Training in Bangalore
How Does Google Flutter Work
Why Google Flutter
Why Choose QuikieApps, for your Video Analytics Solutions?
Mobile App Development Courses in Chennai
Mobile App Development Course Online
Mobile App Development Courses in Bangalore
kaamshakti badhane ki aayurvedik dawa patanjali
agar aap vi aise problem ko face karrhe ho to ye article bahut jaruri hai
Mẫu biệt thự nhà vườn 2 tầng đẹp thơ mộng
Cách bố trí các phòng trong nhà 2 tầng khoa học
Tranh trang trí và phụ kiện Retro
Mẫu tủ quần áo kèm bàn trang điểm bằng gỗ tự nhiên
Mẫu nhà cấp 4 120m2 3 phòng ngủ 1 thờ có nhà tắm đẹp hiện đại