【HDFS API编程】副本系数深度剖析

上一节我们使用Java API操作HDFS文件系统创建了文件a.txt并写入了hello hadoop（回顾：https://www.cnblogs.com/Liuyt-61/p/10739018.html）

我们在终端控制台上使用hadoop fs -ls /hdfsapi/test查看a.txt文件是否创建成功的时候有没有发现一个问题。

[hadoop@hadoop000 ~]$ hadoop fs -ls /hdfsapi/test
Found 1 items
-rw-r--r--   3 hadoop supergroup         14 2019-04-19 16:31 /hdfsapi/test/a.txt

为什么a.txt文件创建了3个副本文件？

我们来到浏览器中查看a.txt文件，也是显示创建了3个副本文件。

我们点开a.txt，block，用户名都是在里面的

问题来了，为什么是3呀？

还记不记得我们在查看HDFS文件内容的时候-put了一个README.txt文件(见最顶上回顾链接)，我们通过浏览器发现它的副本系数是1。

这里为 1 ，我们可能应该知道是我们在/hadoop/etc/hadoop/hdfs-site.xml文件下副本系数设置的为1。

<property>
    <name>dfs.replication</name>
    <value>1</value>
</property>

那么我们使用hdfs shell脚本的方式设置上去他默认走的是我们hdfs-site的配置为 1。那么为什么外面使用Java API传上去的怎么就不是1了？

我们写一个测试类来查看下使用Java API下的副本系数是多少。

public class HDFSApp {

    public static final String HDFS_PATH = "hdfs://hadoop000:8020";
    FileSystem fileSystem = null;
    Configuration configuration = null;

    @Before
    public void setUp() throws Exception{
        System.out.println("setUp-----------");
        configuration = new Configuration();/*
         * 构造一个访问制定HDFS系统的客户端对象
         * 第一个参数：HDFS的URI
         * 第二个参数：客户端制定的配置参数
         * 第三个参数：客户端的身份，说白了就是用户名
         */
        fileSystem = FileSystem.get(new URI(HDFS_PATH),configuration,"hadoop");
    }
    @Test
    public void testReplication(){
        System.out.println(configuration.get("dfs.replication"));
    }
    @After
    public void tearDown(){
        configuration = null;
        fileSystem = null;
        System.out.println("----------tearDown------");
    }
}

输出：
/home/hadoop/app/jdk1.8.0_91/bin/java -ea -Didea.test.cyclic.buffer.size=1048576 -javaagent:/home/hadoop/app/idea-IU-183.5429.30/lib/idea_rt.jar=40319:/home/hadoop/app/idea-IU-183.5429.30/bin -Dfile.encoding=UTF-8 -classpath /home/hadoop/app/idea-IU-183.5429.30/lib/idea_rt.jar:/home/hadoop/app/idea-IU-183.5429.30/plugins/junit/lib/junit-rt.jar:/home/hadoop/app/idea-IU-183.5429.30/plugins/junit/lib/junit5-rt.jar:/home/hadoop/app/jdk1.8.0_91/jre/lib/charsets.jar:/home/hadoop/app/jdk1.8.0_91/jre/lib/deploy.jar:/home/hadoop/app/jdk1.8.0_91/jre/lib/ext/cldrdata.jar:/home/hadoop/app/jdk1.8.0_91/jre/lib/ext/dnsns.jar:/home/hadoop/app/jdk1.8.0_91/jre/lib/ext/jaccess.jar:/home/hadoop/app/jdk1.8.0_91/jre/lib/ext/jfxrt.jar:/home/hadoop/app/jdk1.8.0_91/jre/lib/ext/localedata.jar:/home/hadoop/app/jdk1.8.0_91/jre/lib/ext/nashorn.jar:/home/hadoop/app/jdk1.8.0_91/jre/lib/ext/sunec.jar:/home/hadoop/app/jdk1.8.0_91/jre/lib/ext/sunjce_provider.jar:/home/hadoop/app/jdk1.8.0_91/jre/lib/ext/sunpkcs11.jar:/home/hadoop/app/jdk1.8.0_91/jre/lib/ext/zipfs.jar:/home/hadoop/app/jdk1.8.0_91/jre/lib/javaws.jar:/home/hadoop/app/jdk1.8.0_91/jre/lib/jce.jar:/home/hadoop/app/jdk1.8.0_91/jre/lib/jfr.jar:/home/hadoop/app/jdk1.8.0_91/jre/lib/jfxswt.jar:/home/hadoop/app/jdk1.8.0_91/jre/lib/jsse.jar:/home/hadoop/app/jdk1.8.0_91/jre/lib/management-agent.jar:/home/hadoop/app/jdk1.8.0_91/jre/lib/plugin.jar:/home/hadoop/app/jdk1.8.0_91/jre/lib/resources.jar:/home/hadoop/app/jdk1.8.0_91/jre/lib/rt.jar:/home/hadoop/IdeaProjects/hadoop-train-v2/target/test-classes:/home/hadoop/IdeaProjects/hadoop-train-v2/target/classes:/home/hadoop/.m2/repository/org/apache/hadoop/hadoop-client/2.6.0-cdh5.15.1/hadoop-client-2.6.0-cdh5.15.1.jar:/home/hadoop/.m2/repository/org/apache/hadoop/hadoop-common/2.6.0-cdh5.15.1/hadoop-common-2.6.0-cdh5.15.1.jar:/home/hadoop/.m2/repository/com/google/guava/guava/11.0.2/guava-11.0.2.jar:/home/hadoop/.m2/repository/commons-cli/commons-cli/1.2/commons-cli-1.2.jar:/home/hadoop/.m2/repository/org/apache/commons/commons-math3/3.1.1/commons-math3-3.1.1.jar:/home/hadoop/.m2/repository/xmlenc/xmlenc/0.52/xmlenc-0.52.jar:/home/hadoop/.m2/repository/commons-httpclient/commons-httpclient/3.1/commons-httpclient-3.1.jar:/home/hadoop/.m2/repository/commons-codec/commons-codec/1.4/commons-codec-1.4.jar:/home/hadoop/.m2/repository/commons-io/commons-io/2.4/commons-io-2.4.jar:/home/hadoop/.m2/repository/commons-net/commons-net/3.1/commons-net-3.1.jar:/home/hadoop/.m2/repository/commons-collections/commons-collections/3.2.2/commons-collections-3.2.2.jar:/home/hadoop/.m2/repository/commons-logging/commons-logging/1.1.3/commons-logging-1.1.3.jar:/home/hadoop/.m2/repository/log4j/log4j/1.2.17/log4j-1.2.17.jar:/home/hadoop/.m2/repository/commons-lang/commons-lang/2.6/commons-lang-2.6.jar:/home/hadoop/.m2/repository/commons-configuration/commons-configuration/1.6/commons-configuration-1.6.jar:/home/hadoop/.m2/repository/commons-digester/commons-digester/1.8/commons-digester-1.8.jar:/home/hadoop/.m2/repository/commons-beanutils/commons-beanutils/1.7.0/commons-beanutils-1.7.0.jar:/home/hadoop/.m2/repository/commons-beanutils/commons-beanutils-core/1.8.0/commons-beanutils-core-1.8.0.jar:/home/hadoop/.m2/repository/org/slf4j/slf4j-api/1.7.5/slf4j-api-1.7.5.jar:/home/hadoop/.m2/repository/org/slf4j/slf4j-log4j12/1.7.5/slf4j-log4j12-1.7.5.jar:/home/hadoop/.m2/repository/org/codehaus/jackson/jackson-core-asl/1.8.8/jackson-core-asl-1.8.8.jar:/home/hadoop/.m2/repository/org/codehaus/jackson/jackson-mapper-asl/1.8.8/jackson-mapper-asl-1.8.8.jar:/home/hadoop/.m2/repository/org/apache/avro/avro/1.7.6-cdh5.15.1/avro-1.7.6-cdh5.15.1.jar:/home/hadoop/.m2/repository/com/thoughtworks/paranamer/paranamer/2.3/paranamer-2.3.jar:/home/hadoop/.m2/repository/org/xerial/snappy/snappy-java/1.0.4.1/snappy-java-1.0.4.1.jar:/home/hadoop/.m2/repository/com/google/protobuf/protobuf-java/2.5.0/protobuf-java-2.5.0.jar:/home/hadoop/.m2/repository/com/google/code/gson/gson/2.2.4/gson-2.2.4.jar:/home/hadoop/.m2/repository/org/apache/hadoop/hadoop-auth/2.6.0-cdh5.15.1/hadoop-auth-2.6.0-cdh5.15.1.jar:/home/hadoop/.m2/repository/org/apache/httpcomponents/httpclient/4.2.5/httpclient-4.2.5.jar:/home/hadoop/.m2/repository/org/apache/httpcomponents/httpcore/4.2.4/httpcore-4.2.4.jar:/home/hadoop/.m2/repository/org/apache/directory/server/apacheds-kerberos-codec/2.0.0-M15/apacheds-kerberos-codec-2.0.0-M15.jar:/home/hadoop/.m2/repository/org/apache/directory/server/apacheds-i18n/2.0.0-M15/apacheds-i18n-2.0.0-M15.jar:/home/hadoop/.m2/repository/org/apache/directory/api/api-asn1-api/1.0.0-M20/api-asn1-api-1.0.0-M20.jar:/home/hadoop/.m2/repository/org/apache/directory/api/api-util/1.0.0-M20/api-util-1.0.0-M20.jar:/home/hadoop/.m2/repository/org/apache/curator/curator-framework/2.7.1/curator-framework-2.7.1.jar:/home/hadoop/.m2/repository/org/apache/curator/curator-client/2.7.1/curator-client-2.7.1.jar:/home/hadoop/.m2/repository/org/apache/curator/curator-recipes/2.7.1/curator-recipes-2.7.1.jar:/home/hadoop/.m2/repository/com/google/code/findbugs/jsr305/3.0.0/jsr305-3.0.0.jar:/home/hadoop/.m2/repository/org/apache/htrace/htrace-core4/4.0.1-incubating/htrace-core4-4.0.1-incubating.jar:/home/hadoop/.m2/repository/org/apache/zookeeper/zookeeper/3.4.5-cdh5.15.1/zookeeper-3.4.5-cdh5.15.1.jar:/home/hadoop/.m2/repository/org/apache/commons/commons-compress/1.4.1/commons-compress-1.4.1.jar:/home/hadoop/.m2/repository/org/tukaani/xz/1.0/xz-1.0.jar:/home/hadoop/.m2/repository/org/apache/hadoop/hadoop-hdfs/2.6.0-cdh5.15.1/hadoop-hdfs-2.6.0-cdh5.15.1.jar:/home/hadoop/.m2/repository/org/mortbay/jetty/jetty-util/6.1.26.cloudera.4/jetty-util-6.1.26.cloudera.4.jar:/home/hadoop/.m2/repository/io/netty/netty/3.10.5.Final/netty-3.10.5.Final.jar:/home/hadoop/.m2/repository/io/netty/netty-all/4.0.23.Final/netty-all-4.0.23.Final.jar:/home/hadoop/.m2/repository/xerces/xercesImpl/2.9.1/xercesImpl-2.9.1.jar:/home/hadoop/.m2/repository/xml-apis/xml-apis/1.3.04/xml-apis-1.3.04.jar:/home/hadoop/.m2/repository/org/fusesource/leveldbjni/leveldbjni-all/1.8/leveldbjni-all-1.8.jar:/home/hadoop/.m2/repository/org/apache/hadoop/hadoop-mapreduce-client-app/2.6.0-cdh5.15.1/hadoop-mapreduce-client-app-2.6.0-cdh5.15.1.jar:/home/hadoop/.m2/repository/org/apache/hadoop/hadoop-mapreduce-client-common/2.6.0-cdh5.15.1/hadoop-mapreduce-client-common-2.6.0-cdh5.15.1.jar:/home/hadoop/.m2/repository/org/apache/hadoop/hadoop-yarn-client/2.6.0-cdh5.15.1/hadoop-yarn-client-2.6.0-cdh5.15.1.jar:/home/hadoop/.m2/repository/org/apache/hadoop/hadoop-yarn-server-common/2.6.0-cdh5.15.1/hadoop-yarn-server-common-2.6.0-cdh5.15.1.jar:/home/hadoop/.m2/repository/org/apache/hadoop/hadoop-mapreduce-client-shuffle/2.6.0-cdh5.15.1/hadoop-mapreduce-client-shuffle-2.6.0-cdh5.15.1.jar:/home/hadoop/.m2/repository/org/apache/hadoop/hadoop-yarn-api/2.6.0-cdh5.15.1/hadoop-yarn-api-2.6.0-cdh5.15.1.jar:/home/hadoop/.m2/repository/org/apache/hadoop/hadoop-mapreduce-client-core/2.6.0-cdh5.15.1/hadoop-mapreduce-client-core-2.6.0-cdh5.15.1.jar:/home/hadoop/.m2/repository/org/apache/hadoop/hadoop-yarn-common/2.6.0-cdh5.15.1/hadoop-yarn-common-2.6.0-cdh5.15.1.jar:/home/hadoop/.m2/repository/javax/xml/bind/jaxb-api/2.2.2/jaxb-api-2.2.2.jar:/home/hadoop/.m2/repository/javax/xml/stream/stax-api/1.0-2/stax-api-1.0-2.jar:/home/hadoop/.m2/repository/javax/activation/activation/1.1/activation-1.1.jar:/home/hadoop/.m2/repository/javax/servlet/servlet-api/2.5/servlet-api-2.5.jar:/home/hadoop/.m2/repository/com/sun/jersey/jersey-core/1.9/jersey-core-1.9.jar:/home/hadoop/.m2/repository/com/sun/jersey/jersey-client/1.9/jersey-client-1.9.jar:/home/hadoop/.m2/repository/org/codehaus/jackson/jackson-jaxrs/1.8.8/jackson-jaxrs-1.8.8.jar:/home/hadoop/.m2/repository/org/codehaus/jackson/jackson-xc/1.8.8/jackson-xc-1.8.8.jar:/home/hadoop/.m2/repository/org/apache/hadoop/hadoop-mapreduce-client-jobclient/2.6.0-cdh5.15.1/hadoop-mapreduce-client-jobclient-2.6.0-cdh5.15.1.jar:/home/hadoop/.m2/repository/org/apache/hadoop/hadoop-aws/2.6.0-cdh5.15.1/hadoop-aws-2.6.0-cdh5.15.1.jar:/home/hadoop/.m2/repository/com/amazonaws/aws-java-sdk-bundle/1.11.134/aws-java-sdk-bundle-1.11.134.jar:/home/hadoop/.m2/repository/com/fasterxml/jackson/core/jackson-core/2.2.3/jackson-core-2.2.3.jar:/home/hadoop/.m2/repository/com/fasterxml/jackson/core/jackson-databind/2.2.3/jackson-databind-2.2.3.jar:/home/hadoop/.m2/repository/com/fasterxml/jackson/core/jackson-annotations/2.2.3/jackson-annotations-2.2.3.jar:/home/hadoop/.m2/repository/org/apache/hadoop/hadoop-annotations/2.6.0-cdh5.15.1/hadoop-annotations-2.6.0-cdh5.15.1.jar:/home/hadoop/.m2/repository/junit/junit/4.11/junit-4.11.jar:/home/hadoop/.m2/repository/org/hamcrest/hamcrest-core/1.3/hamcrest-core-1.3.jar com.intellij.rt.execution.junit.JUnitStarter -ideVersion5 -junit4 com.LIuyt.bigdata.hadoop.hdfs.HDFSApp,testReplication
setUp-----------
log4j:WARN No appenders could be found for logger (org.apache.hadoop.metrics2.lib.MutableMetricsFactory).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
3
----------tearDown------

Process finished with exit code 0

看到输出里面没，3 ，这个3怎么来的？

在project中的外部库中找到如图所示hdfs-default.xml文件，我们可以看到里面的副本系数参数配置是3。

因为我们new出来的Configuration configuration = null，它自己就会去加载default.xml，所以它读取出来的是3。

既然我们已经知道了原因，那么我们就可以new之后自己设置一下：

configuration = new Configuration();
configuration.set("dfs.replication","1");

我们再通过同样的方法在a.txt的文件目录下创建一个b.txt来测试一下

    @Test
    public void create() throws Exception{
       //FSDataOutputStream out = fileSystem.create(new Path("/hdfsapi/test/a.txt"));
        FSDataOutputStream out = fileSystem.create(new Path("/hdfsapi/test/b.txt"));
        out.writeUTF("Hello hadoop:replication 1");
        out.flush();//输出走缓冲区，所以先flush
        out.close();
    }

我们再通过终端 -ls查看一下，此时b.txt文件的副本数就是 1 ，再通过浏览器再看一看副本数也是 1 。

[hadoop@hadoop000 ~]$ hadoop fs -ls /hdfsapi/test
Found 2 items
-rw-r--r--   3 hadoop supergroup         14 2019-04-19 16:31 /hdfsapi/test/a.txt
-rw-r--r--   1 hadoop supergroup         28 2019-04-19 16:50 /hdfsapi/test/b.txt

总结：如果是通过命令行传输的就以hdfs-site配置的为准，如果是通过代码传输的就以代码设置的为准。