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Java Basics

Java Intro

Write Once Run Anywhere

  • write once: Java src code (.java file) ==> Javac (complier) () ==> bytecode (.class file)
  • run anywhere: byte code ==> JVM (interpret) ==> machine code (binary file) ==> device (execute)
  • compile once, copy bytecode to any platform having JVM to execute

3 versions [p2]

Edition Note
Java SE (Standard) main for desktop app, inc Java lang fundation, JDBC, nework, thread
Java EE (Enterprise) main for distributed network, core as EJB
Java ME (Embeded) embeded system, e.g. pad，phone

Configuration

搭建Java Env: 安装JDK & JRE，set env_vars

Term Note
JDK Java SE Development Kit, inc compiler (javac), debuger, app launcher, + JDK src code [p7]
JRE run Java program [p7], inc JVM & GC
JVM interpret bytecode to machine code bf each exe (call main())

8 primitive types

  • byte, short, int (default), long (L);
  • float (F/f) & double;
  • char(Unicode char & escape seq), boolean
  • integer literal : 2进制 0b, 8进制 0 [p28], 16进制 0x
  • integer ovreflow: (byte) 129 -> -127
  • (integer) float / double || integer / integer ==> decimal part truncat, -1.2f -> -1, 3/2 = 1
    • want float result in integer devision => casting至少其中一个为float
  • local var no default value
  • ref

Array

2-dim array int[][] tdarr = new int[a][b]

  • a: tdarr.length (行); b: tdarr[i].length (列), can be spec later by tdarr[0] = new int[2]

String

Impl by char[], strVar -> obj (in heap) -> literal (in stack)

Declaration Syntax Note
".." (char-str) ref share if literal exist in stack
new String("..") || expr ref not share (always new a obj in heap)
Check str empty: check null by ==, check "" by equals() (Object.equals(): check by ==)
String methods: length(), toCharArray(), format(..)
Type Note
String literal immutable
StringBuffer thread safe （method synchronized so 同时只有一个线程运行该方法）
StringBuilder more efficient (no thread lock)

Object oriented

Class basics

  • Access ctrl modifier: private (本类), _(本包), protected (子类), public (所有类)
  • [static] {..}: initializer block
    • static: run when obj declare (init as null)
    • non-static: run with new, bf con() (Java compiler copy init blocks into con(…))
  • const: static + final
  • defalut constructor (from compiler): ClassName() {}
  • method: 6 components, method signature: name+param-list, method overload
    • return this; in method [p142]
    • main(): class main entrance [p164]
  • static method(): access static member directly, access instance member by obj ref ..

Inheritance & interface

  • interface: contract btw class & outside-world (promises to provide the behavior of the interface)
    • member auto public static final [p174], cannot define inside a block ..
    • why only method no field: field usually private, method op on internal-state & method usually public — serve as o2o interface with the outside world (data encapsulation)
  • up-cast — same as Parent obj = new Child(); obj.xx(xx must exist in Parent) [p164]
  • instanceof check real type (constructor); null instanceof xx ==> false; instanceof bf down-cast
  • Top class/default parent class: Object
    • get class name: Object.getClass().getName() [p160]
  • abstract: abstract-class vs interface [p174]

Nested Class

  • static nested class: access instance member by obj ref
  • inner class: no own static members (e.g. static {..} & interface) except const
    • local class (block): access enclosing fn param / effectively final local var
      • in static method, can only refer to outer.staticMembers
    • anonymous class (expr of class declare + instanciate, e.g. return, = ~, arg)
      • same member-restriction & accessibity as local classes; no own con();
      • suitable for interface contains >=2 abstract methods, otherwise lambda expr
    • field shadowing (OuterClass.this => obj ref of OuterClass)

Others

  • package — namespace for related class & interface
    • import java.util.*
    • Java SE API: set of pkgs (class lib)
  • GC只能回收new obj — Object.finalize() & System.gc()[p141]

Concurency & Multi-thread

  • 进程(process)：每个独立运行的程序; CUP时间片：每个不同进程轮流执行
  • 当执行一个程序时，JVM自动产生一个线程来运行main() 运行；
  • 在java中实现线程的2种方式：
    • extends java.lang.Thread & override run()
      • run(): 定义线程任务 (无限循环 + bool break 使线程一直运行下去)
      • start(): 启动线程 (call run() internal), 等待系统分配资源 & 运行
    • impl java.lang.Runable (含 run()) — 继承非Thread类并在当前类中实现多线程
      • Thread t = new Thread(new Runnable())
  • 线程的加入 t2.join() in t1.run(): 暂停当前进程，待t执行完毕后继续
  • 线程的暂停 wait()与恢复 nodify()
  • 线程的中断 interrupt()：
    • call sleep() / wait() 进入就绪状态，
    • call interrupt() 跳出run() & throws InterruptedException
  • 线程同步 synchronized：防止多个线程访问资源的冲突，如dirty read — 共享资源上锁，某个时间内只允许一个线程访问该资源
    • synchronized (obj) {..} & synchronized void method(): 放置对共享资源的操作
  • 线程优先级 Thread.MAX/MIN_PRIORITY (1510): 增加/减少线程优先执行的概率，不保证顺序

Internet Communication

  • server: provide init info & info processing; client: fetch info;
  • LAN: Local Area Network; WAN(Wide ~): link LANs, e.g. Internet, the worldwide WAN
  • IP 协议 & IP 地址； IPv4: 4字节/32位 binary; IPv6: 128 bits
  • TCP/IP 协议: based on IP 协议 in Internet (4层)
    • TCP协议：一对一连接，，传输可靠（保证接收且顺序相同), 速度慢（需额外认证），e.g. HTTP, FTP, Telnet
    • UDP协议：一对多，无连接，不保证可靠；数据包之间相对独立（接收顺序不重要），如即时聊天；
  • 端口 port：数据交流的出口（0～65535），e.g. HTTP 80；FTP 21； 套接字socket：连接app & port
  • 相关java类 java.net.*
    • InetAddress: 封装IP地址;
    • SocketAddress: IP + port
    • ServerSocket & Socket: TCP 程序
    • DatagramSocket & DatagramPacket: UDP 程序

TCP程序设计:

  • server new ServerSocket(port) 等待client连接
  • client new Socket(host, port)请求与server连接；
  • server accept() 连接client并return Socket(如一直无client请求连接则阻塞)；
    • server创建新的Socket与client连接，
    • server.accept()return 该(与客户端Socket相连的)Socket；
  • server继续等待新的client连接

note:

  • server 一次与一个client socket连接；默认最大连接容量为50
  • server.accept().getOutputStream() = clientSocket.getInputStream() (server向outputStream写入信息时，client可通过InputStream 来读取，反之亦然)

UDP程序设计:

JDBC API & DB Access

Overview

JDBC API: access tabular data (e.g. Realational DB, flat file or spreadsheet ..), included in JavaSE & JavaEE ..

  • JDBC 4.0 API into 2 pkgs: java.sql & Extension pkg javax.sql;
  • supports both 2-tier & 3-tier processing models for database access

JDBC driver: Drivers that impl JDBC API, dev by DB vendor .., used to communicate with data source ..

  • e.g. MySQL Connector/J (it impl network protocol to MySQL, client connects directly to MySQL) ..
  • contains >= 1 JDBC driver class (class impl interface java.sql.Driver ..)
  • init/load by Class.forName(java.sql.Driver) (when DriverManager first attempts to connect ..)

JDBC Architecture

  • 2-tier model: java app with JDBC driver (client) -> DBMS-proprietary protocol -> DBMS (server);
    3-tire model: Java applet/HTML browser -> HTTP.. -> app server with JDBC.. -> DBMS (DB server)
    • extra middle tier: control access & data updates; simplify the deployment of app

DB concepts ..

  • table (aka relation): a collection of objects of the same type (rows)
  • data in a table related by common keys or concepts

Programming process

Configuration

  • Install DBMS & JDBC driver from DB vendor (e.g. MySQL & Connector/J)
  • spec the JDBC driver class path (path of the JAR file that contains the [JDBC Driver] class specified in driver .., e.g. "com.mysql.cj.jdbc.Driver")

Connect to data source

  • init JDBC driver by Class.forName(java.sql.Driver) (auto-load by DriverManager class for JDBC 4.0+)
  • get connection (to a JDBC driver ..) by DriverManager.getConnection(DB_URL, connProps)
    • DB_URL for MySQL Connector/J: "jdbc:mysql://${DB.HOST}:${DB.PORT}/"

or connect by DataSource (interface in Java EE) ..

  • basic ds: JDBC driver vendor impl DataSource interface & deploy an obj for each data source
  • connection pool ds: ConnectionPoolDataSource obj (impl by JDBC driver vendor) impl connection pooling for DataSource obj (impl by EJB server vendor)
  • distributed transaction ds: XADataSource & DataSource
  • connect by ds.getConnection(..);
  • close conn in finally

Process SQL Stmts

  • 3 statements: Statement, PreparedStatement: precompiled, reusable stmt obj (with diff params by ps.set*(..)) .., CallableStatement & stored procedure ..
  • execute(), executeQuery(query), executeUpdate()
  • batch updates: int [] updateCounts = stmt.executeBatch() (update count in int-arr returned by DBMS, batch list reset to empty)..
  • ResultSet interface ..
    • JDBC driver-spec DatabaseMetaData
    • retrive column value: rs.get*(colIndex/name/alias) & SQL AS in SELECT
    • insert/update/delete rows in rs: rs.moveToInsertRow(), rs.update*(..), rs.insert-/update-/deleteRow(), rs.moveToCurrentRow() ..
      • insertion made immediately af current row ..
  • RowSet interface, JavaBeans Component, scrollable & updatable ..
    • 1 connected JdbcRowSet & 4 disconn (serializable & lightweight, ideal for sending data over a network)
    • CatchedRowSet: super-interface for all disconn (extra crs.acceptChanges(con) to write data to DB)
      • caches data in memory, read/write by a RowSetReader/Writer obj (provided by the assigned SyncProvider instance when crs created)
      • r/w work in bg: get conn, read/write data, close conn
      • ref impl of the default SyncProvider obj: instance of the RIOptimisticProvider impl: (not) write to db if any/no conflict (with no db locks)
      • SyncProviderException & SyncResolver when conflict: compare & choose one
    • JoinRowSet: join mtp disconn rs by jrs.addRowSet(coffees, "SUP_ID");
    • WebRowSet : enable Web Services to send/receive data from db in XML doc (standard for Web Services comm)
      • sets SyncProvider obj to RIXMLProvider impl
      • write-/readXml(..): convert wrs to/from xml
      • contain data + col metadata
    • FilteredRowSet: frs.setFilter(Predicate) (CRUD on current visible rows)
  • transaction & DBMS locks ..
    • default transaction isolation level depends on the DBMS & DatabaseMetaData
    • conn.rollback(SavePoint) ..
  • conn/stmt/rs.close() in finally: close conn also close stmt (good practice to close stmt explicitly) .., close stmt also close rs
  • SQLException: throw when error during interaction with data source ..
    • errorCode: (DB vendor) imp-spec
    • 3 subclass of SQLException JDBC driver-spec might throw ..
  • JDBC 4.1 (Java SE 7+): allow try-resrc stmt .. to auto close conn/stmt/resultSet even exception occur when the try block terminates
  • Adv DB datatype: BLOB, CLOB, XML, Locator (ogical pointer) ..
    • BLOB, CLOB, NCLOB: materialize data as a stream
    • release resrc by obj.free()
    • MySQL not support XML (can store in LONGTEXT), ARRAY (max length) & UDT (DISTINCT, struc) .., RowID (JDBC interface: address to a row in a database table, not a standard SQL type)
    • custom type mapping (from SQL data type to java data type), Datalink & URL ..
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CallableStatement cablStmt = conn.prepareCall("{call pro_GetInfo(?, ?)}"); 
cablStmt.setString(1, "张%"); // 将sql中第一个通配符("?")的值设为“张%” (动态sql)

// update
stmt.executeUpdate("delete..");
ps = conn.prepareStatement("insert/update..");
ps.executeUpdate();

// batch op
ps = conn.prepareStatement("insert/update..");
ps.clearBatch();
for (..) {
	ps.setString(1, records[i][1]);
	ps.setString(2, records[i][1]);
	ps.addBatch();
}
ps.executeBatch();

Refs

Oracle Help Center / Java / Java SE doc / Java Tut / Learning the Java Lang
Javase 8 API
《Java 从入门到精通》(项目案例版) 2017.9