guava Joiner和MapJoiner线程安全

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• {@code Joiner} 和 {@code MapJoiner}　的实例总是 immutable（详见java并发编程实战相关部分）

• [ 1.状态创建后不能被修改． ２．所有的域都是final类型，并且它被正确创建（创建期间没有发生this引用的逸出） ]

• 创建通过静态函数on，内部调用构造函数，对于设置null类型的时候，函数内部会生成一个匿名类，从而重载设置null类型的相关函数．

• 所以他们都是线程安全的． 他们的构造函数都是私有的，必须调用他们的静态方法生成他们的对象（不用反射构造对象情况下）

  public class Chapter2Code {
  
  private final List<String> list = Lists.newArrayList("name", "doctor", "sex", "man", null);
  
  @Rule
  public ExpectedException expectedException = ExpectedException.none();
  
  @Test
  public void test_Joiner() {
  
      Joiner joiner = Joiner.on(",").skipNulls();
  
      assertThat(joiner.join(list), equalTo("name,doctor,sex,man"));
  
      joiner = Joiner.on(",").useForNull("null");
      assertThat(joiner.join(list), equalTo("name,doctor,sex,man,null"));
  
      joiner = Joiner.on("|");
      expectedException.expect(NullPointerException.class);// 如果不处理空指针，会抛出异常．
      joiner.join(list);
  
  }
  
  @Test
  public void test_Joiner_wrong_use() {
  
      Joiner joiner = Joiner.on(",");
      joiner.skipNulls();// 这个方法会生成一个新的对象，对原对象的设置无影响
      expectedException.expect(NullPointerException.class);
      joiner.join(list);
  }
  
  @Test
  public void test_Joiner_appendTo_StringBuilder() {
      Joiner joiner = Joiner.on("|").skipNulls();
      StringBuilder builder = new StringBuilder(256);
  
      joiner.appendTo(builder, list);
  
      assertThat(builder.toString(), equalTo("name|doctor|sex|man"));
      System.out.println(builder.toString());
  }
  
  @Test
  public void test_Joiner_appendTo_File() throws IOException {
      File file = new File("a.txt");
  
      if (!file.exists()) {
          file.createNewFile();
      }
  
      try (FileWriter fileWriter = new FileWriter(file);) {
          Joiner.on("#").skipNulls().appendTo(fileWriter, list);
  
      } catch (Exception e) {
          throw new RuntimeException(e);
      }
  
      List<String> lines = Files.readAllLines(Paths.get("a.txt"));
  
      System.out.println(lines);
      assertThat(Joiner.on("").join(lines), equalTo(Joiner.on("#").skipNulls().join(list)));
  
  }
  
  @Test
  public void test_MapJoiner() {
      Map<String, String> map = Maps.newLinkedHashMap();
      map.put("name", "doctor");
      map.put("sex", "man");
  
      String join = Joiner.on(",").withKeyValueSeparator("=").join(map);
      assertThat(join, equalTo("name=doctor,sex=man"));
  }
  }
  

guava Joiner和MapJoiner线程安全

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原文地址：http://my.oschina.net/doctor2014/blog/387078