ClassPathResource解析

先看Demo:

@Test
public void test() throws IOException {
    Resource res = new ClassPathResource("applicationContext.xml");
    InputStream input = res.getInputStream();
    Assert.assertNotNull(input);
}

內部原始碼：

public ClassPathResource(String path) {
    this(path, (ClassLoader) null);
}


public ClassPathResource(String path, ClassLoader classLoader) {
    Assert.notNull(path, "Path must not be null");
    String pathToUse = StringUtils.cleanPath(path);
    if (pathToUse.startsWith("/")) {
        pathToUse = pathToUse.substring(1);
    }
    this.path = pathToUse;
    this.classLoader = (classLoader != null ? classLoader : ClassUtils.getDefaultClassLoader());
}


public ClassPathResource(String path, Class<?> clazz) {
    Assert.notNull(path, "Path must not be null");
    this.path = StringUtils.cleanPath(path);
    this.clazz = clazz;
}
 

獲取資源內容：

@Override
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;
}
 

所以類獲取資源的方式有兩種

Class獲取和ClassLoader獲取。

兩種方法的Demo和區別：

Demo:

@Test
public void test1() {
    ClassLoader classLoader = Thread.currentThread().getContextClassLoader();

    // ClassLoader.getResource("")獲取的是classpath的根路徑
    System.out.println("a--- " + classLoader.getResource("").getPath());

    // new ClassPathResource()獲取的是空路徑
    System.out.println("b--- " + new ClassPathResource("/").getPath());
    System.out.println("b-1--- " + new ClassPathResource("").getPath());

    // Class.getResource("")獲取的是相對於當前類的相對路徑
    System.out.println("c--- " + this.getClass().getResource("").getPath());

    // Class.getResource("/")獲取的是classpath的根路徑
    System.out.println("d--- " + this.getClass().getResource("/").getPath());
}
 

輸出：

a--- /E:/xie-my-install/devolop/eideaworkspace/ideatest/target/classes/
b--- 
b-1--- 
c--- /E:/xie-my-install/devolop/eideaworkspace/ideatest/target/classes/com/xie/util/
d--- /E:/xie-my-install/devolop/eideaworkspace/ideatest/target/classes/

區別

• Class.getResource("")獲取的是相對於當前類的相對路徑
• Class.getResource("/")獲取的是classpath的根路徑
• ClassLoader.getResource("")獲取的是classpath的根路徑
• new ClassPathResource("")。空路徑，如果沒有指定相對的類名，該類將從類的根路徑開始尋找某個resource，如果指定了相對的類名，則根據指定類的相對路徑來查詢某個resource。
  （如果打成可執行jar包的話，可以使用這種寫法在jar的同級目錄下建立檔案路徑：new ClassPathResource("").getPath() + "/" + UUidUtil.getStrUUID() + ".jpg";）這個可能還沒研究透徹
• 在建立ClassPathResource物件時，我們可以指定是按Class的相對路徑獲取檔案還是按ClassLoader來獲取。