1 JAVA.IO Byte stream

 

inputstream.png

• LineNumberInputStream and StringBufferInputStream Official advice not to use , Recommended LineNumberReader and StringReader Instead of
• ByteArrayInputStream and ByteArrayOutputStream Byte array processing stream , Create a buffer in memory to be used as a stream , Reading data from the cache is better than reading from the storage medium ( Disk ) It's fast

// use ByteArrayOutputStream Temporarily cache data from other channels 
ByteArrayOutputStream data = new ByteArrayOutputStream(1024); //1024 Byte size cache 
data.write(System.in.read()); // Temporary storage of user input data 
// take data To ByteArrayInputStream
ByteArrayInputStream in = new ByteArrayInputStream(data.toByteArray());

• FileInputStream and FileOutputStream access files , Document as InputStream, Read and write the file
• ObjectInputStream and ObjectOutputStream Object flow , The constructor needs to pass in a stream , Realize to JAVA Read and write function of object ; Can be used to serialize , Objects need to be implemented Serializable Interface

//java Object writing 
FileOutputStream fileStream = new FileOutputStream("example.txt");
ObjectOutputStream out = new ObjectOutputStream(fileStream);
Example example = new Example();
out.writeObject(example);
//java Object reading 
FileInputStream fileStream = new FileInputStream("example.txt");
ObjectInputStream in = new ObjectInputStream(fileStream);
Example = (Example) in.readObject();

• PipedInputStream and PipedOutputStream Pipe flow , It is suitable for transferring data in two threads , A thread sends data through a piped output stream , Another thread reads the data through the pipe input stream , Realize the data communication between two threads

// Create a sender object 
Sender sender = new Sender(); // Create a receiver object 
Receiver receiver = new Receiver(); // Get output pipeline flow 
// Get input and output pipeline flow 
PipedOutputStream outputStream = sender.getOutputStream();
PipedInputStream inputStream = receiver.getInputStream();
// Linking two pipes , This is an important step , Connect the input and output streams 
outputStream.connect(inputStream);
sender.start();// Start sender thread 
receiver.start();// Start recipient thread 

• SequenceInputStream The multiple InputStream Merge into one InputStream, Allows applications to combine several input streams continuously

InputStream in1 = new FileInputStream("example1.txt");
InputStream in2 = new FileInputStream("example2.txt");
SequenceInputStream sequenceInputStream = new SequenceInputStream(in1, in2);
// data fetch 
int data = sequenceInputStream.read();

• FilterInputStream and FilterOutputStream Decorator mode is used to add extra functionality to the stream , Subclass construction parameter requires one InputStream/OutputStream

ByteArrayOutputStream out = new ByteArrayOutputStream(2014);
// Data writing , Use DataOutputStream Decorate one InputStream
// Use InputStream Ability to process basic data 
DataOutputStream dataOut = new DataOutputStream(out);
dataOut.writeDouble(1.0);
// data fetch 
ByteArrayInputStream in = new ByteArrayInputStream(out.toByteArray());
DataInputStream dataIn = new DataInputStream(in);
Double data = dataIn.readDouble();

• DataInputStream and DataOutputStream (Filter Subclass of flow ) Additional ability to process various basic types of data for other streams , Such as byte、int、String
• BufferedInputStream and BufferedOutputStream (Filter Subclass of flow ) Add buffering for other streams
• PushBackInputStream (FilterInputStream Subclass ) Push back the input stream , Some of the data read in can be rolled back into the buffer of the input stream
• PrintStream (FilterOutputStream Subclass ) Print stream , The function is similar to System.out.print

2 JAVA.IO Character stream

 

21.png

• From the oriented graph of byte stream and character stream , They correspond to each other , such as CharArrayReader and ByteArrayInputStream
• Conversion of byte stream and character stream ：InputStreamReader Can be InputStream To Reader,OutputStreamReader Can be OutputStream To Writer

//InputStream To Reader
InputStream inputStream = new ByteArrayInputStream(" Program ".getBytes());
InputStreamReader reader = new InputStreamReader(inputStream, StandardCharsets.UTF_8);
//OutputStream To Writer
OutputStream out = new FileOutputStream("example.txt");
OutputStreamWriter writer = new OutputStreamWriter(out);
// Read and write in character units 
writer.write(reader.read(new char[2]));

• difference ： Byte stream read unit is byte , The read unit of character stream is character ; A character consists of bytes , Such as variable word length coding UTF-8 By 1~4 Byte representation

3 Garbled problem and character stream

• Characters are represented by different codes , Its byte length （ The word is long ） It's different . Such as “ cheng ” Of utf-8 Coding format , from [-25][-88][-117] form . and ISO_8859_1 The encoding is a single byte [63]
• Usually, the operation of resources is oriented to byte stream , However, when data resources are converted into bytes according to different byte codes , Their contents are different , It is easy to cause garbled code
• Two kinds of random code encode and decode Inconsistent character encoding used ： Use of resources UTF-8 code , And in code GBK The number of bytes read by byte stream when decoding is opened does not meet the character specified word length ： Characters are made up of bytes , such as “ cheng ” Of utf-8 The format is three bytes ; If in InputStream Read the stream every two bytes , And then to String（java The default encoding is utf-8）, There will be a garbled code （ Half Chinese , Guess what ）

ByteArrayInputStream in = new ByteArrayInputStream(" Good procedure ".getBytes());
byte[] buf = new byte[2]; // Read two bytes of the stream 
in.read(buf); // Reading data 
System.out.println(new String(buf)); // The statement 
---result----
� // The statement 

• Random code scene 1, Know the character encoding of the resource , The corresponding character encoding can be used to decode and solve the problem
• Random code scene 2, All bytes can be read at once , One time coding . But for large file streams , This is unrealistic , So there's a stream of characters
• Byte stream use InputStreamReader、OutputStreamReader Convert to character stream , Where character encoding can be specified , And then it is processed by character , Can solve garbled code

InputStreamReader reader = 
new InputStreamReader(inputStream, StandardCharsets.UTF_8);

4 The concept distinction between character set and character coding

• The relationship between character set and character encoding , The character set is the specification , Character coding is the concrete implementation of the specification ; The character set specifies the unique correspondence between the symbol and the binary code value , However, there is no specific storage method specified ;
• unicode、ASCII、GB2312、GBK All character sets ; among ASCII、GB2312、GBK Both character set and character encoding ; Be careful not to confuse the two ; and unicode The specific implementation of UTF-8,UTF-16,UTF-32
• The first ASCII Code uses a byte （8bit） To specify the character and binary mapping relationship , standard ASCII The code specifies 128 Characters , In the English world , It's enough . But Chinese , How to map Japanese and other characters ？ So other, larger character sets appear
• unicode（ Unified character set ）, In the early days it was used 2 individual byte Express 1 Characters , The entire character set can hold 65536 Characters . However, it is still not enough , So it expanded to 4 individual byte Represents a character , At present, the scope of support is U+010000~U+10FFFF
• unicode It's a mistake to say it's two bytes ;UTF-8 It's variable length , Need to use 1~4 Byte store ;UTF-16 It's usually two bytes （U+0000~U+FFFF Range ）, If you encounter two bytes, you can't save it , Then use 4 Bytes ; and UTF-32 It's fixed four bytes
• unicode The character represented , Will use “U+” start , It's followed by a hexadecimal number , Such as “ word ” The code of is U+5B57
• UTF-8 Coding and unicode Character set

Range  Unicode(Binary) UTF-8 code (Binary) UTF-8 code byte length  U+0000~U+007F 00000000 00000000 00000000 0XXXXXXX 0XXXXXX 1 U+0080~U+07FF 00000000 00000000 00000YYY YYXXXXXX 110YYYYY 10XXXXXX 2 U+0800~U+FFFF 00000000 00000000 ZZZZYYYY YYXXXXXX 1110ZZZZ 10YYYYYY 10XXXXXX 3 U+010000~U+10FFFF 00000000 000AAAZZ ZZZZYYYY YYXXXXXX 11110AAA 10ZZZZZZ 10YYYYYY 10XXXXXX 4

• The program is divided into internal code and external code ,java The default encoding for is UTF-8, In fact, it refers to the outer code ; Internal codes tend to use fixed length codes , A principle of alignment with memory , Easy to handle . Outer codes tend to use variable length codes , Variable length code encodes common characters into short codes , Long encoding of rare characters , Save storage space and transmission bandwidth
• JDK8 String , It's using char[] To store characters ,char It's two bytes in size , Which uses UTF-16 code （ Internal code ）. and unicode Chinese characters specified in U+0000~U+FFFF Inside , Therefore use char（UTF-16 code ） There will be no garbled code when storing Chinese
• JDK9 after , String uses byte[] Array to store , Because there are some characters, one char It can't be saved , Such as emoji emoticons , Using bytes to store strings is easier to expand
• JDK9, If the content of the string is ISO-8859-1/Latin-1 character （1 Characters 1 byte ）, Then use ISO-8859-1/Latin-1 Encoding storage strings , Otherwise use UTF-16 Coded storage array （2 or 4 Bytes ）

System.out.println(Charset.defaultCharset()); // Output java Default encoding 
for (byte item : " Program ".getBytes(StandardCharsets.UTF_16)) {
System.out.print("[" + item + "]");
}
System.out.println("");
for (byte item : " Program ".getBytes(StandardCharsets.UTF_8)) {
System.out.print("[" + item + "]");
}
----result----
UTF-8 //java Default encoding UTF-8
[-2][-1][122][11][94][-113] //UTF_16：6 Bytes ？
[-25][-88][-117][-27][-70][-113] //UTF_8：6 Bytes normal 

• “ Program ” Of UTF-16 Code is output 6 Bytes , Two more bytes , What's going on here ？ Try one more character output

for (byte item : " cheng ".getBytes(StandardCharsets.UTF_16)) {
 System.out.print("[" + item + "]");
}
---result--
[-2][-1][122][11]

• It can be seen that UTF-16 There are too many bytes encoded [-2][-1] Two bytes , Hex is 0xFEFF. And it's used to identify the coding order is Big endian still Little endian. In characters ' in ' For example , its unicode Hex is 4E2D, When the storage 4E before ,2D After , Namely Big endian;2D before ,4E After , Namely Little endian.FEFF Indicates that the storage adopts Big endian,FFFE Said the use of Little endian
• Why? UTF-8 There is no problem with byte order ？ Personal view , because UTF-8 It's getting longer , From the head of the first byte 0、110、1110、11110 Determine whether the next few bytes are needed to form a character , Use Big endian Easy to read processing , The reverse is not easy to deal with , Therefore, it is mandatory to use Big endian
• In fact, I feel UTF-16 It can be mandatory to use Big endian; But there is a historical problem ...

5 URI A brief introduction to the concept

• Given the java.io To manipulate the resource flow ; But for the resources of the network , How to open it , How to locate it ？ answer URI-URL
• URI The full name is Uniform Resource Identifier Uniform resource identifiers
• Popular said , It's a string similar to the ID card number. , It's just that it's used to identify resources ( Such as ： mailing address , Host name , Documents, etc. )
• URI Have specific rules : [scheme]:[scheme-specific-part][#fragment] Further subdivision can be expressed as [scheme]:[//authority][/path][?query][#fragment], The mode specific part is authority and path、query; and authority It can be seen as a domain name , Such as www.baidu.com The ultimate subdivision is [scheme]:[//host:port][/path][?query][#fragment], Just as like as two peas.
• Pattern specific parts (scheme-specific-part) The form depends on the pattern , and URI The common patterns of are as follows ftp：FTP The server file： Files on local disk http： Use Hypertext Transfer Protocol mailto： Email address telnet： be based on Telnet Service connection for Java Some non-standard customization patterns are also widely used in , Such as rmi、jar、jndi、doc、jdbc etc.
• stay java in URI Abstract to java.net.URI class , Here are several common construction methods

// according to str Generate URI
public URI(String str) throws URISyntaxException
public URI(String scheme, String authority,
String path, String query, String fragment)throws URISyntaxException
public static URI create(String str) // call URI(String str) 

• JAVA.URI Common operation methods of

public String getScheme() // Access mode 
public String getSchemeSpecificPart()// Get mode specific parts 
public String getFragment() // Get fragment identifier 
// The above three methods are universal 
public String getAuthority() // Authorized institutions , Such as www.baidu.com
public String getHost() // Get host part , Such as 127.0.0.1
public int getPort() // Such as 8080
public String getPath() // Positioning path 
public String getQuery() // Query criteria 

6 URL Concepts and URL The difference between

• URL The full name is Uniform Resource Location, Uniform resource locator
• URL Namely URI Subset , In addition to identifying resources , It also provides a path to find the resource ; stay Java Class library ,URI Class does not contain any methods to access resources , Its only function is to parse , and URL Class to open a flow to a resource
• Of the same genus URI Subset of URN( Unified resource name ), Identify resource name only , It does not specify how to locate resources ; Such as ：mailto：clswcl@gmail.com It's a kind of URN, I know it's a mailbox , But I don't know how to find the location
• Popular is ,URN Tell you, there is a place called Guangzhou , But it didn't say how , You can take the train , You can also fly ;URL Will tell you to fly to Guangzhou , And the other URL It's about taking the train
• URL General grammar rules of

 agreement :// Host name : port / route ? Inquire about # fragment
[protocol]:[//host:port][/path][?query][#fragment]

• URL Construction method of 、 Access method

// be based on URL Pattern construction URL example 
public URL(String spec) throws MalformedURLException
// among file amount to path、query and fragment Three parts 
public URL(String protocol, String host, int port, String file) throws MalformedURLException
// According to the class loader URL
URL systemResource = ClassLoader.getSystemResource(String name)
Enumeration<URL> systemResources = ClassLoader.getSystemResources(String name)
URL resource = Main.class.getResource(String name)
Enumeration<URL> resources = Main.class.getClassLoader().getResources(String name)

• adopt URL Operation function for getting resource data

public final InputStream openStream() throws java.io.IOException
public URLConnection openConnection() throws java.io.IOException
public final Object getContent() throws java.io.IOException

7 Spring.Resource And Spring Resource acquisition method

• Talking about resources , I have to talk about it Spring Access to resources , There are two common ones adopt Resource The subclass of interface gets resources through ResourceLoader Interface to get resources
• Spring.Resource List of resource operation functions

// Determine whether the resource exists 
boolean exists(); //
// Returns the current resource corresponding to URL, If it cannot be resolved, an exception will be thrown ; Such as ByteArrayResource It can't be resolved into one URL
URL getURL() throws IOException;
// Returns the current resource corresponding to URI
URI getURI() throws IOException;
// Returns the current resource corresponding to File
File getFile() throws IOException;
// Return the corresponding ReadableByteChannel
default ReadableByteChannel readableChannel() throws IOException

• introduce Resource Use of related subclasses
• 1 FileSystemResource： Getting resources through the file system

Resource resource = new FileSystemResource("D:/example.txt");
File file= new File("example.txt");
Resource resource2 = new FileSystemResource(file);

• 2 ByteArrayResource： obtain byte The resource represented by the array be based on ByteArrayInputStream And byte array implementation , The application scenario is similar ByteArrayInputStream, cache byte[] resources
• 3 ClassPathResource： Get the resources under the classpath

//ClassPathResource.java Three attributes of 
private final String path;
// Use Class or ClassLoader load resources 
private ClassLoader classLoader;
private Class<?> clazz;
--- Usage mode ----
Resource resource = new ClassPathResource("test.txt");

• 4 InputStreamResource： Receive one InputStream object , Gets the resource encapsulated by the input stream
• 5 ServletContextResourse： load ServletContext In the environment （ be relative to Web Application root ） Path resources , Resources obtained
• 6 UrlResource： adopt URL visit http Resources and FTP Resources, etc

8 ResourceLoader Access to resources

 

resource.png

• ResourceLoader It's for shielding Resource The concrete realization of , Unified access to resources . You can start from ResourceLoader load ClassPathResource, Can also be loaded FileSystemResource etc.

public interface ResourceLoader {
// Resources loaded from the classpath by default Prefix : "classpath:", obtain ClassPathResource
String CLASSPATH_URL_PREFIX = ResourceUtils.CLASSPATH_URL_PREFIX;
Resource getResource(String location);

• ResourceLoader Interface default classpath The resources under the path are loaded

public interface ResourcePatternResolver extends ResourceLoader {
// All paths are loaded by default （ Include jar package ） The following file ,"classpath*:", obtain ClassPathResource
String CLASSPATH_ALL_URL_PREFIX = "classpath*:";

• ResourcePatternResolver Files under all paths are loaded by default , get ClassPathResource;classpath： Only in class Search under class path ; and classpath*： Will scan all JAR Package and class Files in the classpath

//Ant Style expression com/smart/**/*.xml 
ResourcePatternResoler resolver = new PathMatchingResourcePatternResolver();
Resource resources[] = resolver.getResources("com/smart/**/*.xml");
// ApplicationContext ctx 
//FileSystemResource resources 
Resource template = ctx.getResource("file:///res.txt");
//UrlResource resources 
Resource template = ctx.getResource("https://my.cn/res.txt");

• ResourceLoader Method getResource Of locationPattern You can set the resource pattern prefix to get non ClassPathResource resources ,locationPattern Support Ant style

Prefix   Example   describe  classpath: classpath:config.xml Load from classpath file: file:///res.txt Load from file system FileSystemResource http: http://my.cn/res.txt load UrlResource

9 JAVA.Properties Get to know

• Properties yes java Configuration processing class with ;Properties Two ways to load resources

public class Properties extends Hashtable<Object,Object>{
.... // According to the Reader perhaps InputStream load properties The contents of the document 
public synchronized void load(Reader reader) throws IOException
public synchronized void load(InputStream inStream) throws IOException

• Properties Read configuration sample code

//res.properties
username = root
password = password
------- Code example -------------
InputStream input = ClassLoader.getSystemResourceAsStream("res.properties");
Properties prop = new Properties();
prop.load(inputStream); // according to inputStream Load Resource
String username = prop.getProperty("username");

10 yml Read configuration resources

• Ordinary java If the project needs to be read yml Can be introduced jackson-dataformat-yaml, and springboot Default configuration support yml The read

<dependency>
<groupId>com.fasterxml.jackson.dataformat</groupId>
<artifactId>jackson-dataformat-yaml</artifactId>
<version>2.9.5</version>

• be based on jackson-dataformat-yaml Yes yml Read configuration resources

//res.yml To configure 
name: chen
params:
url: http://www.my.com
---------- Code example ---------------
InputStream input = ClassLoader.getSystemResourceAsStream("res.yml");
Yaml yml = new Yaml();
Map map = new Yaml().loadAs(input, LinkedHashMap.class);; // according to inputStream Load Resource 
String name = MapUtils.getString(map,"name"); // chen
//url: http://www.my.com

11 Closing resources gracefully ,try-with-resource Grammar and lombok@Cleanup

• Opening resources requires corresponding closing , But we often forget to shut down resources , Or it's messy to close resources in multiple places , Is there a simple way to close it ？
• To automatically close the resource class AutoCloseable Interface and coordination try-with-resource Grammar sugar use

public class YSOAPConnection implements AutoCloseable {
private SOAPConnection connection;
public static YSOAPConnection open(SOAPConnectionFactory soapConnectionFactory) throws SOAPException {
YSOAPConnection ySoapConnection = new YSOAPConnection();
SOAPConnection connection = soapConnectionFactory.createConnection();
ySoapConnection.setConnection(connection);
return ySoapConnection;
}
public SOAPMessage call(SOAPMessage request, Object to) throws SOAPException {
return connection.call(request, to);
}
@Override
public void close() throws SOAPException {
if (connection != null) { connection.close(); }
}
}

// Examples of automatically closed resource classes 
try (YSOAPConnection soapConnection=YSOAPConnection.open(soapConnectionFactory)){
SOAPMessage soapResponse = soapConnection.call(request, endpoint);
...// Data manipulation 
} catch (Exception e) {
log.error(e.getMessage(), e);
...
}

• lombok annotation @Cleanup, Called at the end of the object's life cycle public void close(); Object needs to implement AutoCloseable Interface

import lombok.Cleanup;
@Cleanup // @Cleanup Use 
YSOAPConnection soapConnection=YSOAPConnection.open(soapConnectionFactory)

12 Resource not closed , What's the worst result

• JDK The native resource class of is not closed , And it won't last forever .JVM With the help of finalize Turn it off automatically , for example FileInputStream

//FileInputStream.java - JDK8
//jdk8 Of FileInputStream Rewrote finalize, Ensure that the resources opened before object recycling are closed 
protected void finalize () throws IOException {
if (guard != null) {
guard.warnIfOpen();
}
if ((fd != null) && (fd != FileDescriptor.in)) {
close();
}
}

• stay JDK9 after , use Cleaner Mechanism instead finalize Mechanism ;Cleaner Objects that are automatically recycled by the mechanism also need to be implemented AutoCloseable Interface ;Cleaner Is based on PhantomReference Realized ; Students interested in implementation details , You can refer to the relevant documents by yourself
• But use JDK Resources provided by the closure mechanism , It takes a long time to shut down the resource manually . According to the test , Use try-with-resources close resource , And let the garbage collector recycle it in time 12 nanosecond . While using finalizer Mechanism , Time increased to 550 nanosecond
• Not closing resources in time , It takes up resources , Affect the execution of other threads ; such as linux File resources for ,linux The maximum number of files a process can open by default is 1024（ There are plenty of them 2048, This value is configurable ）; If a thread holds more than a dozen file resources , Also wait 550 Nanosecond finalizer Mechanism releases resources , The other threads in the same process are waiting to die

author ：clswcl
link ：https://juejin.im/post/6856266775022174222
source ： Nuggets