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---
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title: Java
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created: 2020-04-08
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updated: 2020-04-08
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type: summary
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tags: [tech, reference]
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external: https://github.com/wahyd4/knowledge/blob/master/categories/java.md
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---
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## Java GC
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### GC types
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- Serial GC
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- Parallel GC
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- CMS GC
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- G1 GC
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### Object type
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- Young generation minor GC
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- Old generation full GC
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- System.gc()
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## JVM
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- Method Area
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- Class name, Constant, static Constant, Class object
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- Heap
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- Object instance, array
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- young generation
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- Old generation
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- VM stack
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- local variable
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- dynamic links
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- operation stacks
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- Native method stack
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- program counter
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### JVM insight
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## Spring
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- Spring boot
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- Rest controller
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- Jpa template
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- Spring scheduler
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- Annotation bean
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- Spring cloud
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- Consol
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- Zuul
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- Cloud config
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- Spring cloud zookeeper( discovery and configuration)
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- Spring ecurity
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## ORM
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- Hibernate
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- validation
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- JPA
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- Sprint jpa tempalte
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- Mybatis
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## Build
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- Gradle
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- groovy task
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- Maven
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- XML plugins
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- Maven mirror
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- [https://maven-central.storage.googleapis.com](https://maven-central.storage.googleapis.com)
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- [http://maven.aliyun.com/nexus/content/groups/public/](http://maven.aliyun.com/nexus/content/groups/public/)
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- Ant
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- task based
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## Composition or inheritance
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Think of containment as a has a relationship. A car "has an" engine, a person "has a" name, etc.
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Think of inheritance as an is a relationship. A car "is a" vehicle, a person "is a" mammal, etc.
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## Useful tips
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### Iterate map
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- for (Map.Entry<String, String> entry : map.entrySet()) { }
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### List to array
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- int[] arr= new int[list.length](); list.toArray(arr);
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### Object
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- immutable
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- object that can’t be modified after it’s created
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- Don’t allow subclass override methods, class to be final
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- Don’t provide setter method
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- Make all field private and final
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- mutable
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### Default Timezone
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- TimeZone.setDefault(TimeZone.getTimeZone("GMT+8"))
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### [generics](https://appliedgo.net/generics/)
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- Why we need?
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- e.g we have sort method, and different type(n) of objects. then you have to write n kind of search methods.
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### event listener
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- [https://stackoverflow.com/questions/6270132/create-a-custom-event-in-java](https://stackoverflow.com/questions/6270132/create-a-custom-event-in-java)
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### Try with resource
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The try-with-resources statement is a try statement that declares one or more resources. A resource is an object that must be closed after the program is finished with it. The try-with-resources statement ensures that each resource is closed at the end of the statement. Any object that implements java.lang.AutoCloseable, which includes all objects which implement java.io.Closeable, can be used as a resource.
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In short no need add manual `finally` block to close resources.
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```java
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try (InputStream caInput = new ByteArrayInputStream(decode)) {
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// Generate the CA Certificate from the raw resource file
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Certificate ca = CertificateFactory.getInstance("X.509").generateCertificate(caInput);
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// Load the key store using the CA
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KeyStore keyStore = KeyStore.getInstance(KeyStore.getDefaultType());
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keyStore.load(null, null);
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keyStore.setCertificateEntry("ca", ca);
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// Initialize the TrustManager with this CA
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TrustManagerFactory tmf = TrustManagerFactory.getInstance(TrustManagerFactory.getDefaultAlgorithm());
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tmf.init(keyStore);
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// Create an SSL context that uses the created trust manager
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SSLContext sslContext = SSLContext.getInstance("TLS");
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sslContext.init(null, tmf.getTrustManagers(), new SecureRandom());
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return sslContext.getSocketFactory();
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} catch (Exception ex) {
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logger.error(ex.getMessage());
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throw new RuntimeException(ex);
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}
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```
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## JAVA SE
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### Java Collection
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#### HashMap and HashTable
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- `Hashtable` is synchronized, whereas `HashMap` is not. This makes HashMap better for non-threaded applications, as unsynchronized Objects typically perform better than synchronized ones.
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- `Hashtable` does not allow null keys or values. `HashMap` allows one null key and any number of null values.
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- One of HashMap's subclasses is `LinkedHashMap`, so in the event that you'd want predictable iteration order (which is insertion order by default), you could easily swap out the HashMap for a LinkedHashMap. This wouldn't be as easy if you were using Hashtable.
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- `HashMap` is not thread-safe, if you do want to have thread-safe one then use `Collections.synchronizedMap()`,
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- HashTable is thread-safe, but is treated as legacy.
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### modifier
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* public
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* no modifier
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* protected
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* priivate
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### Java modifier diagram
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### NIO
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- Channels
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- A channel is a kind of stream, from the channel data can be read into a buffer.
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- channel implementations
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- FileChannel
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- reads data from and to files
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- DatagramChannel
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- read and write data over the network via UDP
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- SocketChannel
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- read and write data over the network visa TCP
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- ServerSocketChannel
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- allows you to listen for incoming TCP connections, like a web server. For each incoming connection a SocketChannel is created.
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- Buffer
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- code sample
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```java
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RandomAccessFile aFile = new RandomAccessFile("data/nio-data.txt", "rw");
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FileChannel inChannel = aFile.getChannel();
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//create buffer with capacity of 48 bytes
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ByteBuffer buf = ByteBuffer.allocate(48);
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**int bytesRead = inChannel.read(buf**); //read into buffer.
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while (bytesRead != -1) {
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**buf.flip**(); //make buffer ready for read
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while(buf.hasRemaining()){
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System.out.print((char) **buf.get**()); // read 1 byte at a time
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}
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**buf.clear**(); //make buffer ready for writing
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bytesRead = inChannel.read(buf);
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}
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aFile.close();
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```
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- properties
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- capacity
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- position
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- limit
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- Steps for buffer to read and write data.
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- write data into buffer
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- call buffer.flip()
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- read data out of the buffer
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- call buffer.clear() or buffer.compact()
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- Non-blocking IO
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- Selectors
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- A selector allows a single thread to handle multiple Channels
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- [http://tutorials.jenkov.com/java-nio/index.html](http://tutorials.jenkov.com/java-nio/index.html)
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### SSL
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#### Java use TLSv1.2 by default since Java 8
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When you saw some error like this `SSL handshake exception: “Algorithm constraints check failed: MD5withRSA”`, probably you used `MD5` as message digest when generate a CA. You should regenerate a new CA using `sha1`, because the `MD5` has been found to be insecure.
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Some more info <https://stackoverflow.com/questions/21218217/ssl-handshake-exception-algorithm-constraints-check-failed-md5withrsa>
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### Monotonic clock
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`Note`: You can go to the `go` page to check out more background knowledge
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```java
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long startTime = System.nanoTime();
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// ... the code being measured ...
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long estimatedTime = System.nanoTime() - startTime;
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```
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Some code which uses the wall clock
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```java
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long start = System.currentTimeMillis();
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// Do something exceptional
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long elapsed = System.currentTimeMillis() - start;
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```
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