Java Network Programming & Web Communication – Comprehensive Bullet Notes

Spam Check Functionality in Java

Purpose: Detect unsolicited/harmful messages (email, SMS, forum posts).
Core Techniques
- Content analysis: scan body/subject/metadata for spam patterns (excessive links, "win money now", obfuscation like “fr33”).
- Header inspection: verify sender, forged headers, spam-listed IPs.
- Blacklist look-ups: compare sender IP/domain against RBLs.
- Bayesian filtering: compute $P(\text{spam}|\text{terms})$ using prior corpora.
- Machine-learning / AI: train classifiers on thousands of labelled messages.
- Pattern matching & heuristics: regex for repeated words, odd formatting.
- Scoring & thresholds: aggregate rule scores; flag if total > cutoff.
Typical Deployments
- Email providers (Gmail, Yahoo) – phishing & junk blocking.
- Web forums/comment sections – prevent bot posts.
- SMS gateways – block fraud texts.

Processing Server Log Files in Java

Pipeline
- Collection: pull logs from web/app/db servers.
- Parsing & tokenisation: split into fields (timestamp, IP, URL, status…).
- Filtering & aggregation: group by severity (INFO/WARN/ERROR), detect bursts.
- Pattern matching: regex for repeated 401/403, failed logins.
- Real-time monitoring: stream logs for intrusion/failure alerts.
- Storage & archiving: DB, flat-file, or cloud object store.
- Reporting & visualisation: dashboards, time-series graphs.
Use-cases
- Security monitoring (unauthorised logins, DDoS, malware).
- Performance (slow queries, high CPU, bottlenecks).
- Debugging (crash traces).
- Compliance & auditing.

URL / URI Essentials

URL syntax: scheme://username:password@host:port/path?query#fragment
• Base URI + Relative URI → $\text{base}.resolve(\text{relative})$ to get absolute.
URI vs URL vs URN: URI = generic identifier; URL = locator; URN = name.
Components
- Scheme (http, https, ftp…)
- Authority (user, pass, host, port)
- Path
- Query string (key=value&…)
- Fragment (#section)
Opaque URI: non-hierarchical; scheme-specific part treated as one chunk.

Spammers – Why Block Them

Security (malware, phishing).
Privacy (unsolicited data collection).
Bandwidth & server costs.
Reputation damage to domains/IPs.
User-experience clutter.

x-www-form-urlencoded

Encodes POST body as query-string style: name=Rustam&age=25.
Java utilities: URLEncoder, URLDecoder, HttpClient’s FormBodyPublisher, etc.

Proxy Concepts

Types
- Forward, Reverse, Transparent, Anonymous, Caching.
Use-cases: security isolation, privacy, caching, access-control.
Java flags
- -Dhttp.proxyHost, -Dhttp.proxyPort, -Dhttp.nonProxyHosts="*.example.com|localhost".
java.net.Proxy / ProxySelector
- Proxy.Type = HTTP, SOCKS, DIRECT.
- ProxySelector.getDefault() pulls OS proxy; can override.
Authenticator & PasswordAuthentication
- Central mechanism for Basic/Digest/proxy auth; credentials stored as char[].

HTTP Overview

Versions: 0.9 → 1.0 → 1.1 → 2 (binary, multiplexing) → 3 (QUIC).
Request line + headers + optional body; server returns status line + headers + body.
Common Methods: GET, POST, PUT, DELETE, HEAD, OPTIONS, PATCH, TRACE.
Status Codes
- 1xx Info (100,101)
- 2xx Success (200,201,204)
- 3xx Redirect (301,302,304)
- 4xx Client (400,401,403,404)
- 5xx Server (500,502,503)
Keep-Alive
- Header: Connection: keep-alive, Keep-Alive: timeout=10, max=100.
- Reuses single TCP connection → lower latency.

HTTP Request Body & Examples

POST (create), PUT (replace), PATCH (partial), DELETE.
JSON example shown with 201/200 responses.

Cookies & Java Handling

Cookie attributes: Expires, Max-Age, Domain, Path, Secure, HttpOnly, SameSite.
Risks: hijacking, XSS, CSRF.
Java classes
- CookieHandler, CookieManager, CookieStore, custom overrides.

URLConnection / HttpURLConnection

Steps: new URL → openConnection → set headers/method → (write body) → read response → disconnect.
Features: multi-protocol (HTTP, HTTPS, FTP), input/output streams, auth, caching.
Comparison: URL = identifier; URLConnection = communication.

MIME Types

Define file formats (e.g., text/html, application/json, image/png).
Java: Files.probeContentType(path) guesses via extension or magic numbers.

Web Caching

Layers: browser, proxy, CDN, app cache.
Cache hit vs miss; improves load time, reduces bandwidth.
Java demo: ResponseCache.setDefault(...) — first call miss, second hit.
Security risks with stale/private data if mis-configured.

Security Risks with URLConnection & Mitigations

Plain HTTP → sniffing. Use HTTPS.
MITM → validate certs; set HSTS.
Missing headers → add Authorization, Cache-Control: no-store.
URL injection → validate trusted domain.
Leaked API keys → never hardcode; encrypt.

Socket Communication Fundamentals

Why: IPC, client-server, real-time, remote control.
Properties
- Address = IP+port.
- TCP (reliable) vs UDP (fast, no guarantee).
- Full-duplex; blocking/non-blocking modes.
Lifecycle (TCP): socket() → bind() → listen()/connect() → accept() → send/recv → close().
Advantages: speed, scalability, reliability (TCP).
Disadvantages: complexity, security exposure, overhead, network dependency.

Telnet for Testing

telnet host port → manually type HTTP GET / HTTP/1.1 to see raw response.

ServerSocket & Multithreaded Servers

ServerSocket(port) listens; each accept() returns new Socket.
Multithreaded: spawn ClientHandler thread per connection → concurrent service.
Exceptions: BindException, SocketException, IOException, SecurityException.
Binary data example: server sends timestamp as bytes, client decodes.

Socket Options Cheat-Sheet

TCP_NODELAY – disable Nagle, reduce latency.
SO_LINGER – control blocking close.
SO_TIMEOUT – read timeout.
SO_RCVBUF / SO_SNDBUF – buffer sizes.
SO_KEEPALIVE – periodic probes.
SO_REUSEADDR – quick rebinding.
SO_OOBINLINE – inline urgent data.
IP_TOS / setTrafficClass() – QoS bits.

Secure Sockets (SSL/TLS)

Package: javax.net.ssl — SSLSocket, SSLServerSocket, SSLContext.
Steps: load keystore → init context → create secure socket → handshake events via HandshakeCompletedListener.
Configure cipher suites (e.g., TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384).

Java Logging Basics

Purposes: debugging, performance, security audit, ops insight.
Levels (java.util.logging)
- $1000$ SEVERE, $900$ WARNING, $800$ INFO, $700$ CONFIG, $500$ FINE, $400$ FINER, $300$ FINEST.
Avoid logging sensitive/PII data.

Java NIO (Non-blocking I/O)

Building blocks
- Channels (SocketChannel, ServerSocketChannel, FileChannel, DatagramChannel).
- Buffers (ByteBuffer, CharBuffer…) with capacity, limit, position, mark.
- Selectors monitor many channels with one thread.
Workflow
1. Channel non-blocking.
2. Register with Selector for OP_READ/WRITE/ACCEPT/CONNECT.
3. Loop selector.select() → iterate ready keys.
4. Read/write via buffers; handle partial operations.
Buffer operations
- put(), flip(), get(), clear(), compact(), duplicate(), slice().
Pros: high scalability, fewer threads.
Cons: complex callbacks, back-pressure handling, not always faster for small loads.

Selector / SelectionKey Quick Table

Operation	Meaning
OP_ACCEPT	Server socket ready to accept
OP_CONNECT	Client ready to finish connect
OP_READ	Data available to read
OP_WRITE	Channel ready to write

UDP Programming in Java

Connectionless, no guarantee, max datagram $65{,}535\ \text{bytes}$ .
Classes
- DatagramSocket (send/recv) — send(packet), receive(packet), setSoTimeout().
- DatagramPacket holds byte[], length, destination.
- DatagramChannel (NIO, non-blocking) — send(ByteBuffer, addr), receive(ByteBuffer).
Socket options: SO_TIMEOUT, SO_RCVBUF, SO_SNDBUF, SO_REUSEADDR, SO_BROADCAST, IP_TOS.
Common echo server/client example; suitable for DNS, streaming, games.

IP Multicasting

Address range 224.0.0.0 – 239.255.255.255.
Routers use IGMP; protocols like PIM for routing.
Java MulticastSocket → joinGroup(addr) / leaveGroup(addr); send to group.
Use for live video, conferencing, distributed events.

Streaming Mode & Chunked Transfer in Java

setChunkedStreamingMode(1024) – send HTTP body in 1 KB chunks → low memory footprint.

WHOIS via Sockets

TCP port 43.
Simple Java client connects to whois.verisign-grs.com, sends domain, prints registry info.

Summary Relationships & Implications

Effective spam/log processing + proxies + secure sockets ensure security and performance of Java network apps.
Understanding HTTP intricacies (methods, headers, keep-alive, status codes, cookies) is foundational for web APIs.
NIO & multithreaded servers offer scalable architectures; choose TCP vs UDP vs Multicast based on reliability vs speed.
Logging & MIME handling complement network code by adding observability and correct content negotiation.