java 资源管理_Java资源管理以及在Spring中的应用

我不是女神ヾ 2022-11-05 08:53 4阅读 0赞

1. 资源种类

1.1 文件资源

XML文件

Properties文件

File file1 = new File(""); //默认为用户路径

System.out.println(file1.getAbsolutePath());

System.out.println(System.getProperty("user.dir"));//用户当前工作路径

File file = new File("D:\\\\projects\\\\idea\_workspace\\\\spring-boot-lesson\\\\spring-java-resources\\\\src\\\\main\\\\resources\\\\application.properties");

URL fileURL = file.toURI().toURL();

System.out.println(fileURL);

以上代码得出结果：

file:/D:/projects/idea\_workspace/spring-boot-lesson/spring-java-resources/src/main/resources/application.properties

它是file协议(URL)

读取Properties文件事例

public class PropertiesDemo \{

public static void main(String\[\] args) throws Exception \{

Properties properties = new Properties();

//1. 绝对路径方式，不妥

// File file = new File("D:\\\\projects\\\\idea\_workspace\\\\spring-boot-lesson\\\\spring-java-resources\\\\src\\\\main\\\\resources\\\\application.properties");

// properties.load(new FileReader(file));

//2. 可以读取class path路径下的,太多，不可取

// System.out.println(System.getProperty("java.class.path"));

//3. 利用ClassLoader，读取class path路径下的资源(相对路径)

//Maven默认的Resource目录就会放到class path下

ClassLoader classLoader = Thread.currentThread().getContextClassLoader();

//读取class path路径下的文件

InputStream is = classLoader.getResourceAsStream("application.properties");

properties.load(is);

System.out.println(properties.getProperty("spring.application.name"));

1.2 网络资源Socket

HTTP

FTP

利用org.springframework.web.client.RestTemplate 访问HTTP资源：

//https://start.spring.io/

// Spring RestTemplate

RestTemplate restTemplate = new RestTemplate();

InputStream inputStreamFromRestTemplate =

restTemplate.execute("https://start.spring.io/",

HttpMethod.GET,

request -> \{

\},

response -> \{

return response.getBody();

);

System.out.println(inputStreamFromRestTemplate);

利用java.net.URL访问HTTP资源：

URL url = new URL("https://start.spring.io/");

InputStream inputStreamFromURL = url.openStream();

System.out.println(inputStreamFromURL); //inputStreamFromRestTemplate对应的实现类与inputStreamFromURL对应的实现类相同

1.3 类路径资源

2. Java URL 协议扩展

2.1 URL协议扩展过程

Java URL协议具有以下层次关系：

java.net.URL\#setURLStreamHandlerFactory -> java.net.URLStreamHandlerFactory\#createURLStreamHandler -> java.net.URLStreamHandler\#openConnection(java.net.URL)

由上面的代码InputStream inputStreamFromURL = url.openStream();可得知，需要通过java.net.URL\#openStream得到InputStream对象。

public final InputStream openStream() throws java.io.IOException \{

return openConnection().getInputStream();

由java.net.URL\#openConnection()方法可得知，

public URLConnection openConnection() throws java.io.IOException \{

return handler.openConnection(this);

需要得到 java.net.URLConnection ，需要使用java.net.URLStreamHandler\#openConnection(java.net.URL) 方法，而java.net.URLStreamHandler是一个抽象类，比如有以下的具体实现：

//策略模式的实现

//REMIND: decide whether to allow the "null" class prefix or not.

//packagePrefixList += "sun.net.www.protocol";

//模式 URLStreamHandler = sun.net.www.protocol.$\{protocol\}.Handler

file协议：URLStreamHandler = sun.net.www.protocol.file.Handler

http协议：URLStreamHandler = sun.net.www.protocol.http.Handler

https协议：URLStreamHandler = sun.net.www.protocol.https.Handler

jar协议：URLStreamHandler = sun.net.www.protocol.jar.Handler

ftp协议：URLStreamHandler = sun.net.www.protocol.ftp.Handler

通过 java.net.URLConnection\#getInputStream 得知，需要得到具体的InputStream，则需要扩展URLConnection，比如 sun.net.www.protocol.http.HttpURLConnection ，它继承自 java.net.HttpURLConnection。

public synchronized InputStream getInputStream() throws IOException \{

this.connecting = true;

SocketPermission var1 = this.URLtoSocketPermission(this.url);

if(var1 != null) \{

try \{

return (InputStream)AccessController.doPrivileged(new PrivilegedExceptionAction() \{

public InputStream run() throws IOException \{

return HttpURLConnection.this.getInputStream0();

\}, (AccessControlContext)null, new Permission\[\]\{var1\});

\} catch (PrivilegedActionException var3) \{

throw (IOException)var3.getException();

\} else \{

return this.getInputStream0();

URL -> URLConnection -> URLStreamHandler -> InputStream

2.2 URL协议扩展原理

针对每个协议的不同，JDK利用策略模式进行相应的实现，每个协议都有相应的Handler。

比如URL其中一个构造方法：

public URL(String protocol, String host, int port, String file,

URLStreamHandler handler) throws MalformedURLException \{

if (handler != null) \{

SecurityManager sm = System.getSecurityManager();

if (sm != null) \{

// check for permission to specify a handler

checkSpecifyHandler(sm);

....

// Note: we don't do validation of the URL here. Too risky to change

// right now, but worth considering for future reference. -br

if (handler == null &&

(handler = getURLStreamHandler(protocol)) == null) \{

throw new MalformedURLException("unknown protocol: " + protocol);

this.handler = handler;

上面构造方法得知，每个URL都可以跟进不同的Handler构造。

需要扩展URL协议的话，需要按照约定的Handler包命名规则：

sun.net.www.protocol(前缀) + 协议($\{protocol\}) + Handler

思考一个问题：Spring Boot打成Jar包，是如何访问到Jar包中的资源？

Spring boot其实对 java.net.URLStreamHandlerFactory做了扩展

org.springframework.boot.context.embedded.tomcat.SslStoreProviderUrlStreamHandlerFactory

org.springframework.boot.context.embedded.jetty.JasperInitializer.WarUrlStreamHandlerFactory

JDK对java.net.URLStreamHandlerFactory的默认实现

sun.misc.Launcher.Factory 它的实现如下：

private static class Factory implements URLStreamHandlerFactory \{

private static String PREFIX = "sun.net.www.protocol";

private Factory() \{

public URLStreamHandler createURLStreamHandler(String var1) \{

String var2 = PREFIX + "." + var1 + ".Handler";

try \{

Class var3 = Class.forName(var2);

return (URLStreamHandler)var3.newInstance();

\} catch (ReflectiveOperationException var4) \{

throw new InternalError("could not load " + var1 + "system protocol handler", var4);

通过查看factory调用的源码可知，Spring Boot读取Jar文件中的资源是跟java.net.URLClassLoader\#URLClassLoader(java.net.URL\[\], java.lang.ClassLoader, java.net.URLStreamHandlerFactory)有关的。

static class AppClassLoader extends URLClassLoader \{

.....

AppClassLoader(URL\[\] var1, ClassLoader var2) \{

super(var1, var2, Launcher.factory);

.....

在读取Jar包中资源的时候，他已经设置了相应的URLStreamHandlerFactory(即：java.net.URL\#setURLStreamHandlerFactory )，进而创建相应的URLStreamHandler，即：sun.net.www.protocol.jar.Handler。

3. Spring资源管理

Spring中资源管理是Resource接口，org.springframework.core.io.Resource,他有以下一些实现：

org.springframework.core.io.FileSystemResource

org.springframework.core.io.PathResource

org.springframework.core.io.ClassPathResource

通过 org.springframework.core.io.ResourceLoader\#getResource 得到相应Resource对象，而 org.springframework.core.io.ResourceLoade有很多种实现，其默认实现为：org.springframework.core.io.DefaultResourceLoader。

如下案例说明ResourceLoader的用法：

public class ResourceDemo \{

public static void main(String\[\] args) throws Exception \{

// Resource

// FileSystemResource

// ClasspathResource

DefaultResourceLoader resourceLoader = new DefaultResourceLoader();

Resource resource = resourceLoader.getResource("classpath:/application.properties");

InputStream inputStream = resource.getInputStream();

String content = StreamUtils.copyToString(inputStream, Charset.forName("UTF-8"));

System.out.println(content);

如上代码发现，需要获取InputStream，则需要通过ResourceLoader获取Resource，然后调用getInputStream得到。具体如下：

org.springframework.core.io.DefaultResourceLoader\#getResource 方法：

@Override

public Resource getResource(String location) \{

Assert.notNull(location, "Location must not be null");

for (ProtocolResolver protocolResolver : this.protocolResolvers) \{

Resource resource = protocolResolver.resolve(location, this);

if (resource != null) \{

return resource;

if (location.startsWith("/")) \{

return getResourceByPath(location);

else if (location.startsWith(CLASSPATH\_URL\_PREFIX)) \{

return new ClassPathResource(location.substring(CLASSPATH\_URL\_PREFIX.length()), getClassLoader());

else \{

try \{

// Try to parse the location as a URL...

URL url = new URL(location);

return new UrlResource(url);

catch (MalformedURLException ex) \{

// No URL -> resolve as resource path.

return getResourceByPath(location);

上述代码可知，如果url前缀为“classpath:”的时候，则通过 org.springframework.core.io.ClassPathResource\#ClassPathResource(java.lang.String, java.lang.ClassLoader) 实现，其getInputStream方法如下：

public InputStream getInputStream() throws IOException \{

InputStream is;

if (this.clazz != null) \{

is = this.clazz.getResourceAsStream(this.path);

else if (this.classLoader != null) \{

is = this.classLoader.getResourceAsStream(this.path);

else \{

is = ClassLoader.getSystemResourceAsStream(this.path);

if (is == null) \{

throw new FileNotFoundException(getDescription() + " cannot be opened because it does not exist");

return is;

如果不是“classpath:”开头的，则由 org.springframework.core.io.UrlResource 实现，此时底层getInputStream实现就是委派给了java.net.URLConnection实现。如下代码所示：