昨天看到了AbstractFileSystem，也知道应用访问文件是通过FileContext这个类，今天来看这个类的源代码，先看下这个类老长的注释说明
1 /** 2 * The FileContext class provides an interface to the application writer for 3 * using the Hadoop file system. 4 * It provides a set of methods for the usual operation: create, open, 5 * list, etc 6 * 7 * <p> 8 * <b> *** Path Names *** </b> 9 * <p> 10 * 11 * The Hadoop file system supports a URI name space and URI names. 12 * It offers a forest of file systems that can be referenced using fully 13 * qualified URIs. 14 * Two common Hadoop file systems implementations are 15 * <ul> 16 * <li> the local file system: file:///path 17 * <li> the hdfs file system hdfs://nnAddress:nnPort/path 18 * </ul> 19 * 20 * While URI names are very flexible, it requires knowing the name or address 21 * of the server. For convenience one often wants to access the default system 22 * in one's environment without knowing its name/address. This has an 23 * additional benefit that it allows one to change one's default fs 24 * (e.g. admin moves application from cluster1 to cluster2). 25 * <p> 26 * 27 * To facilitate this, Hadoop supports a notion of a default file system. 28 * The user can set his default file system, although this is 29 * typically set up for you in your environment via your default config. 30 * A default file system implies a default scheme and authority; slash-relative 31 * names (such as /for/bar) are resolved relative to that default FS. 32 * Similarly a user can also have working-directory-relative names (i.e. names 33 * not starting with a slash). While the working directory is generally in the 34 * same default FS, the wd can be in a different FS. 35 * <p> 36 * Hence Hadoop path names can be one of: 37 * <ul> 38 * <li> fully qualified URI: scheme://authority/path 39 * <li> slash relative names: /path relative to the default file system 40 * <li> wd-relative names: path relative to the working dir 41 * </ul> 42 * Relative paths with scheme (scheme:foo/bar) are illegal. 43 * 44 * <p> 45 * <b>****The Role of the FileContext and configuration defaults****</b> 46 * <p> 47 * The FileContext provides file namespace context for resolving file names; 48 * it also contains the umask for permissions, In that sense it is like the 49 * per-process file-related state in Unix system. 50 * These two properties 51 * <ul> 52 * <li> default file system i.e your slash) 53 * <li> umask 54 * </ul> 55 * in general, are obtained from the default configuration file 56 * in your environment, (@see {@link Configuration}). 57 * 58 * No other configuration parameters are obtained from the default config as 59 * far as the file context layer is concerned. All file system instances 60 * (i.e. deployments of file systems) have default properties; we call these 61 * server side (SS) defaults. Operation like create allow one to select many 62 * properties: either pass them in as explicit parameters or use 63 * the SS properties. 64 * <p> 65 * The file system related SS defaults are 66 * <ul> 67 * <li> the home directory (default is "/user/userName") 68 * <li> the initial wd (only for local fs) 69 * <li> replication factor 70 * <li> block size 71 * <li> buffer size 72 * <li> encryptDataTransfer 73 * <li> checksum option. (checksumType and bytesPerChecksum) 74 * </ul> 75 * 76 * <p> 77 * <b> *** Usage Model for the FileContext class *** </b> 78 * <p> 79 * Example 1: use the default config read from the $HADOOP_CONFIG/core.xml. 80 * Unspecified values come from core-defaults.xml in the release jar. 81 * <ul> 82 * <li> myFContext = FileContext.getFileContext(); // uses the default config 83 * // which has your default FS 84 * <li> myFContext.create(path, ...); 85 * <li> myFContext.setWorkingDir(path) 86 * <li> myFContext.open (path, ...); 87 * </ul> 88 * Example 2: Get a FileContext with a specific URI as the default FS 89 * <ul> 90 * <li> myFContext = FileContext.getFileContext(URI) 91 * <li> myFContext.create(path, ...); 92 * ... 93 * </ul> 94 * Example 3: FileContext with local file system as the default 95 * <ul> 96 * <li> myFContext = FileContext.getLocalFSFileContext() 97 * <li> myFContext.create(path, ...); 98 * <li> ... 99 * </ul> 100 * Example 4: Use a specific config, ignoring $HADOOP_CONFIG 101 * Generally you should not need use a config unless you are doing 102 * <ul> 103 * <li> configX = someConfigSomeOnePassedToYou. 104 * <li> myFContext = getFileContext(configX); // configX is not changed, 105 * // is passed down 106 * <li> myFContext.create(path, ...); 107 * <li>... 108 * </ul> 109 * 110 */ 111 112 @InterfaceAudience.Public 113 @InterfaceStability.Evolving /*Evolving for a release,to be changed to Stable */ 114 public class FileContext {