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content="Nicksxs's Blog" itemprop=name><meta content="Nicksxs 的技术博客,深入 Java 运行时与生产诊断、低成本大模型推理和 Agent 工程实践。" itemprop=description></span><span hidden itemprop=post itemscope itemtype=http://schema.org/CreativeWork><meta content="聊聊 Java 中绕不开的 Synchronized 关键字 | Nicksxs's Blog" itemprop=name><meta content="Synchronized 关键字在 Java 的并发体系里也是非常重要的一个内容,首先比较常规的是知道它使用的方式,可以锁对象,可以锁代码块,也可以锁方法,看一个简单的 demo" itemprop=description></span><header class=post-header><h1 itemprop="name headline" class=post-title>聊聊 Java 中绕不开的 Synchronized 关键字</h1><div class=post-meta-container><div class=post-meta><span class=post-meta-item><span class=post-meta-item-icon><i class="far fa-calendar"></i> </span><span class=post-meta-item-text>发表于</span> <time itemprop="dateCreated datePublished" title="创建时间:2021-06-20 23:10:34" datetime=2021-06-20T23:10:34+08:00>2021-06-20</time> </span><span class=post-meta-item><span class=post-meta-item-icon><i class="far fa-folder"></i> </span><span class=post-meta-item-text>分类于</span> <span itemprop=about itemscope itemtype=http://schema.org/Thing><a href=/categories/Java/ itemprop=url rel=index><span itemprop=name>Java</span></a> </span></span><span class="post-meta-item cloudflare-page-counter" data-title="聊聊 Java 中绕不开的 Synchronized 关键字" data-increment=true data-path=/2021/06/20/%E8%81%8A%E8%81%8A-Java-%E4%B8%AD%E7%BB%95%E4%B8%8D%E5%BC%80%E7%9A%84-Synchronized-%E5%85%B3%E9%94%AE%E5%AD%97/ title=阅读次数><span class=post-meta-item-icon><i class="far fa-eye"></i> </span><span class=post-meta-item-text>阅读次数:</span> <span class=cloudflare-page-counter-count></span></span></div><div class=post-description>Synchronized 关键字在 Java 的并发体系里也是非常重要的一个内容,首先比较常规的是知道它使用的方式,可以锁对象,可以锁代码块,也可以锁方法,看一个简单的 demo</div></div></header><div class=post-body itemprop=articleBody><p>Synchronized 关键字在 Java 的并发体系里也是非常重要的一个内容,首先比较常规的是知道它使用的方式,可以锁对象,可以锁代码块,也可以锁方法,看一个简单的 demo<figure class="highlight java"><table><tr><td class=gutter><pre><span class=line>1</span><br><span class=line>2</span><br><span class=line>3</span><br><span class=line>4</span><br><span class=line>5</span><br><span class=line>6</span><br><span class=line>7</span><br><span class=line>8</span><br><span class=line>9</span><br><span class=line>10</span><br><span class=line>11</span><br><span class=line>12</span><br><span class=line>13</span><br><span class=line>14</span><br><span class=line>15</span><br><span class=line>16</span><br><span class=line>17</span><br></pre><td class=code><pre><span class=line><span class=keyword>public</span> <span class=keyword>class</span> <span class="title class_">SynchronizedDemo</span> {</span><br><span class=line></span><br><span class=line> <span class=keyword>public</span> <span class=keyword>static</span> <span class=keyword>void</span> <span class="title function_">main</span><span class=params>(String[] args)</span> {</span><br><span class=line> <span class=type>SynchronizedDemo</span> <span class=variable>synchronizedDemo</span> <span class=operator>=</span> <span class=keyword>new</span> <span class="title class_">SynchronizedDemo</span>();</span><br><span class=line> synchronizedDemo.lockMethod();</span><br><span class=line> }</span><br><span class=line></span><br><span class=line> <span class=keyword>public</span> <span class=keyword>synchronized</span> <span class=keyword>void</span> <span class="title function_">lockMethod</span><span class=params>()</span> {</span><br><span class=line> System.out.println(<span class=string>"here i'm locked"</span>);</span><br><span class=line> }</span><br><span class=line></span><br><span class=line> <span class=keyword>public</span> <span class=keyword>void</span> <span class="title function_">lockSynchronizedDemo</span><span class=params>()</span> {</span><br><span class=line> <span class=keyword>synchronized</span> (<span class=built_in>this</span>) {</span><br><span class=line> System.out.println(<span class=string>"here lock class"</span>);</span><br><span class=line> }</span><br><span class=line> }</span><br><span class=line>}</span><br></pre></table></figure><p>然后来查看反编译结果,其实代码(日光)之下并无新事,即使是完全不懂的也可以通过一些词义看出一些意义<figure class="highlight java"><table><tr><td class=gutter><pre><span class=line>1</span><br><span class=line>2</span><br><span class=line>3</span><br><span class=line>4</span><br><span class=line>5</span><br><span class=line>6</span><br><span class=line>7</span><br><span class=line>8</span><br><span class=line>9</span><br><span class=line>10</span><br><span class=line>11</span><br><span class=line>12</span><br><span class=line>13</span><br><span class=line>14</span><br><span class=line>15</span><br><span class=line>16</span><br><span class=line>17</span><br><span class=line>18</span><br><span class=line>19</span><br><span class=line>20</span><br><span class=line>21</span><br><span class=line>22</span><br><span class=line>23</span><br><span class=line>24</span><br><span class=line>25</span><br><span class=line>26</span><br><span class=line>27</span><br><span class=line>28</span><br><span class=line>29</span><br><span class=line>30</span><br><span class=line>31</span><br><span class=line>32</span><br><span class=line>33</span><br><span class=line>34</span><br><span class=line>35</span><br><span class=line>36</span><br><span class=line>37</span><br><span class=line>38</span><br><span class=line>39</span><br><span class=line>40</span><br><span class=line>41</span><br><span class=line>42</span><br><span class=line>43</span><br><span class=line>44</span><br><span class=line>45</span><br><span class=line>46</span><br><span class=line>47</span><br><span class=line>48</span><br><span class=line>49</span><br><span class=line>50</span><br><span class=line>51</span><br><span class=line>52</span><br><span class=line>53</span><br><span class=line>54</span><br><span class=line>55</span><br><span class=line>56</span><br><span class=line>57</span><br><span class=line>58</span><br><span class=line>59</span><br><span class=line>60</span><br><span class=line>61</span><br><span class=line>62</span><br><span class=line>63</span><br><span class=line>64</span><br><span class=line>65</span><br><span class=line>66</span><br><span class=line>67</span><br><span class=line>68</span><br><span class=line>69</span><br><span class=line>70</span><br><span class=line>71</span><br><span class=line>72</span><br><span class=line>73</span><br><span class=line>74</span><br><span class=line>75</span><br><span class=line>76</span><br><span class=line>77</span><br><span class=line>78</span><br><span class=line>79</span><br><span class=line>80</span><br><span class=line>81</span><br><span class=line>82</span><br><span class=line>83</span><br><span class=line>84</span><br><span class=line>85</span><br><span class=line>86</span><br><span class=line>87</span><br><span class=line>88</span><br><span class=line>89</span><br><span class=line>90</span><br><span class=line>91</span><br><span class=line>92</span><br><span class=line>93</span><br><span class=line>94</span><br><span class=line>95</span><br><span class=line>96</span><br><span class=line>97</span><br><span class=line>98</span><br><span class=line>99</span><br><span class=line>100</span><br><span class=line>101</span><br><span class=line>102</span><br><span class=line>103</span><br><span class=line>104</span><br><span class=line>105</span><br><span class=line>106</span><br><span class=line>107</span><br><span class=line>108</span><br><span class=line>109</span><br><span class=line>110</span><br><span class=line>111</span><br><span class=line>112</span><br><span class=line>113</span><br><span class=line>114</span><br><span class=line>115</span><br><span class=line>116</span><br><span class=line>117</span><br><span class=line>118</span><br><span class=line>119</span><br><span class=line>120</span><br><span class=line>121</span><br><span class=line>122</span><br><span class=line>123</span><br><span class=line>124</span><br><span class=line>125</span><br><span class=line>126</span><br><span class=line>127</span><br><span class=line>128</span><br><span class=line>129</span><br><span class=line>130</span><br></pre><td class=code><pre><span class=line> Last modified <span class=number>2021</span>年<span class=number>6</span>月<span class=number>20</span>日; size <span class=number>729</span> bytes</span><br><span class=line> MD5 checksum dd9c529863bd7ff839a95481db578ad9</span><br><span class=line> Compiled from <span class=string>"SynchronizedDemo.java"</span></span><br><span class=line><span class=keyword>public</span> <span class=keyword>class</span> <span class="title class_">SynchronizedDemo</span></span><br><span class=line> minor version: <span class=number>0</span></span><br><span class=line> major version: <span class=number>53</span></span><br><span class=line> flags: (<span class=number>0x0021</span>) ACC_PUBLIC, ACC_SUPER</span><br><span class=line> this_class: #<span class=number>2</span> <span class=comment>// SynchronizedDemo</span></span><br><span class=line> super_class: #<span class=number>9</span> <span class=comment>// java/lang/Object</span></span><br><span class=line> interfaces: <span class=number>0</span>, fields: <span class=number>0</span>, methods: <span class=number>4</span>, attributes: <span class=number>1</span></span><br><span class=line>Constant pool:</span><br><span class=line> #<span class=number>1</span> = Methodref #<span class=number>9.</span>#<span class=number>22</span> <span class=comment>// java/lang/Object."&LTinit>":()V</span></span><br><span class=line> #<span class=number>2</span> = Class #<span class=number>23</span> <span class=comment>// SynchronizedDemo</span></span><br><span class=line> #<span class=number>3</span> = Methodref #<span class=number>2.</span>#<span class=number>22</span> <span class=comment>// SynchronizedDemo."&LTinit>":()V</span></span><br><span class=line> #<span class=number>4</span> = Methodref #<span class=number>2.</span>#<span class=number>24</span> <span class=comment>// SynchronizedDemo.lockMethod:()V</span></span><br><span class=line> #<span class=number>5</span> = Fieldref #<span class=number>25.</span>#<span class=number>26</span> <span class=comment>// java/lang/System.out:Ljava/io/PrintStream;</span></span><br><span class=line> #<span class=number>6</span> = String #<span class=number>27</span> <span class=comment>// here i\'m locked</span></span><br><span class=line> #<span class=number>7</span> = Methodref #<span class=number>28.</span>#<span class=number>29</span> <span class=comment>// java/io/PrintStream.println:(Ljava/lang/String;)V</span></span><br><span class=line> #<span class=number>8</span> = String #<span class=number>30</span> <span class=comment>// here lock class</span></span><br><span class=line> #<span class=number>9</span> = Class #<span class=number>31</span> <span class=comment>// java/lang/Object</span></span><br><span class=line> #<span class=number>10</span> = Utf8 &LTinit></span><br><span class=line> #<span class=number>11</span> = Utf8 ()V</span><br><span class=line> #<span class=number>12</span> = Utf8 Code</span><br><span class=line> #<span class=number>13</span> = Utf8 LineNumberTable</span><br><span class=line> #<span class=number>14</span> = Utf8 main</span><br><span class=line> #<span class=number>15</span> = Utf8 ([Ljava/lang/String;)V</span><br><span class=line> #<span class=number>16</span> = Utf8 lockMethod</span><br><span class=line> #<span class=number>17</span> = Utf8 lockSynchronizedDemo</span><br><span class=line> #<span class=number>18</span> = Utf8 StackMapTable</span><br><span class=line> #<span class=number>19</span> = Class #<span class=number>32</span> <span class=comment>// java/lang/Throwable</span></span><br><span class=line> #<span class=number>20</span> = Utf8 SourceFile</span><br><span class=line> #<span class=number>21</span> = Utf8 SynchronizedDemo.java</span><br><span class=line> #<span class=number>22</span> = NameAndType #<span class=number>10</span>:#<span class=number>11</span> <span class=comment>// "&LTinit>":()V</span></span><br><span class=line> #<span class=number>23</span> = Utf8 SynchronizedDemo</span><br><span class=line> #<span class=number>24</span> = NameAndType #<span class=number>16</span>:#<span class=number>11</span> <span class=comment>// lockMethod:()V</span></span><br><span class=line> #<span class=number>25</span> = Class #<span class=number>33</span> <span class=comment>// java/lang/System</span></span><br><span class=line> #<span class=number>26</span> = NameAndType #<span class=number>34</span>:#<span class=number>35</span> <span class=comment>// out:Ljava/io/PrintStream;</span></span><br><span class=line> #<span class=number>27</span> = Utf8 here i\<span class=string>'m locked</span></span><br><span class=line><span class=string> #28 = Class #36 // java/io/PrintStream</span></span><br><span class=line><span class=string> #29 = NameAndType #37:#38 // println:(Ljava/lang/String;)V</span></span><br><span class=line><span class=string> #30 = Utf8 here lock class</span></span><br><span class=line><span class=string> #31 = Utf8 java/lang/Object</span></span><br><span class=line><span class=string> #32 = Utf8 java/lang/Throwable</span></span><br><span class=line><span class=string> #33 = Utf8 java/lang/System</span></span><br><span class=line><span class=string> #34 = Utf8 out</span></span><br><span class=line><span class=string> #35 = Utf8 Ljava/io/PrintStream;</span></span><br><span class=line><span class=string> #36 = Utf8 java/io/PrintStream</span></span><br><span class=line><span class=string> #37 = Utf8 println</span></span><br><span class=line><span class=string> #38 = Utf8 (Ljava/lang/String;)V</span></span><br><span class=line><span class=string>{</span></span><br><span class=line><span class=string> public SynchronizedDemo();</span></span><br><span class=line><span class=string> descriptor: ()V</span></span><br><span class=line><span class=string> flags: (0x0001) ACC_PUBLIC</span></span><br><span class=line><span class=string> Code:</span></span><br><span class=line><span class=string> stack=1, locals=1, args_size=1</span></span><br><span class=line><span class=string> 0: aload_0</span></span><br><span class=line><span class=string> 1: invokespecial #1 // Method java/lang/Object."&LTinit>":()V</span></span><br><span class=line><span class=string> 4: return</span></span><br><span class=line><span class=string> LineNumberTable:</span></span><br><span class=line><span class=string> line 5: 0</span></span><br><span class=line><span class=string></span></span><br><span class=line><span class=string> public static void main(java.lang.String[]);</span></span><br><span class=line><span class=string> descriptor: ([Ljava/lang/String;)V</span></span><br><span class=line><span class=string> flags: (0x0009) ACC_PUBLIC, ACC_STATIC</span></span><br><span class=line><span class=string> Code:</span></span><br><span class=line><span class=string> stack=2, locals=2, args_size=1</span></span><br><span class=line><span class=string> 0: new #2 // class SynchronizedDemo</span></span><br><span class=line><span class=string> 3: dup</span></span><br><span class=line><span class=string> 4: invokespecial #3 // Method "&LTinit>":()V</span></span><br><span class=line><span class=string> 7: astore_1</span></span><br><span class=line><span class=string> 8: aload_1</span></span><br><span class=line><span class=string> 9: invokevirtual #4 // Method lockMethod:()V</span></span><br><span class=line><span class=string> 12: return</span></span><br><span class=line><span class=string> LineNumberTable:</span></span><br><span class=line><span class=string> line 8: 0</span></span><br><span class=line><span class=string> line 9: 8</span></span><br><span class=line><span class=string> line 10: 12</span></span><br><span class=line><span class=string></span></span><br><span class=line><span class=string> public synchronized void lockMethod();</span></span><br><span class=line><span class=string> descriptor: ()V</span></span><br><span class=line><span class=string> flags: (0x0021) ACC_PUBLIC, ACC_SYNCHRONIZED</span></span><br><span class=line><span class=string> Code:</span></span><br><span class=line><span class=string> stack=2, locals=1, args_size=1</span></span><br><span class=line><span class=string> 0: getstatic #5 // Field java/lang/System.out:Ljava/io/PrintStream;</span></span><br><span class=line><span class=string> 3: ldc #6 // String here i\'m locked</span></span><br><span class=line><span class=string> 5: invokevirtual #7 // Method java/io/PrintStream.println:(Ljava/lang/String;)V</span></span><br><span class=line><span class=string> 8: return</span></span><br><span class=line><span class=string> LineNumberTable:</span></span><br><span class=line><span class=string> line 13: 0</span></span><br><span class=line><span class=string> line 14: 8</span></span><br><span class=line><span class=string></span></span><br><span class=line><span class=string> public void lockSynchronizedDemo();</span></span><br><span class=line><span class=string> descriptor: ()V</span></span><br><span class=line><span class=string> flags: (0x0001) ACC_PUBLIC</span></span><br><span class=line><span class=string> Code:</span></span><br><span class=line><span class=string> stack=2, locals=3, args_size=1</span></span><br><span class=line><span class=string> 0: aload_0</span></span><br><span class=line><span class=string> 1: dup</span></span><br><span class=line><span class=string> 2: astore_1</span></span><br><span class=line><span class=string> 3: monitorenter</span></span><br><span class=line><span class=string> 4: getstatic #5 // Field java/lang/System.out:Ljava/io/PrintStream;</span></span><br><span class=line><span class=string> 7: ldc #8 // String here lock class</span></span><br><span class=line><span class=string> 9: invokevirtual #7 // Method java/io/PrintStream.println:(Ljava/lang/String;)V</span></span><br><span class=line><span class=string> 12: aload_1</span></span><br><span class=line><span class=string> 13: monitorexit</span></span><br><span class=line><span class=string> 14: goto 22</span></span><br><span class=line><span class=string> 17: astore_2</span></span><br><span class=line><span class=string> 18: aload_1</span></span><br><span class=line><span class=string> 19: monitorexit</span></span><br><span class=line><span class=string> 20: aload_2</span></span><br><span class=line><span class=string> 21: athrow</span></span><br><span class=line><span class=string> 22: return</span></span><br><span class=line><span class=string> Exception table:</span></span><br><span class=line><span class=string> from to target type</span></span><br><span class=line><span class=string> 4 14 17 any</span></span><br><span class=line><span class=string> 17 20 17 any</span></span><br><span class=line><span class=string> LineNumberTable:</span></span><br><span class=line><span class=string> line 17: 0</span></span><br><span class=line><span class=string> line 18: 4</span></span><br><span class=line><span class=string> line 19: 12</span></span><br><span class=line><span class=string> line 20: 22</span></span><br><span class=line><span class=string> StackMapTable: number_of_entries = 2</span></span><br><span class=line><span class=string> frame_type = 255 /* full_frame */</span></span><br><span class=line><span class=string> offset_delta = 17</span></span><br><span class=line><span class=string> locals = [ class SynchronizedDemo, class java/lang/Object ]</span></span><br><span class=line><span class=string> stack = [ class java/lang/Throwable ]</span></span><br><span class=line><span class=string> frame_type = 250 /* chop */</span></span><br><span class=line><span class=string> offset_delta = 4</span></span><br><span class=line><span class=string>}</span></span><br><span class=line><span class=string>SourceFile: "SynchronizedDemo.java"</span></span><br></pre></table></figure><p>其中<code>lockMethod</code>中可以看到是通过 <code>ACC_SYNCHRONIZED</code> flag 来标记是被 synchronized 修饰,前面的 ACC 应该是 access 的意思,并且通过 <code>ACC_PUBLIC</code> 也可以看出来他们是同一类访问权限关键字来控制的,而修饰类则是通过<code>3: monitorenter</code>和<code>13: monitorexit</code>来控制并发,这个是原来就知道,后来看了下才知道修饰方法是不一样的,但是在前期都比较诟病是 synchronized 的性能,像 monitor 也是通过操作系统的<code>mutex lock</code>互斥锁来实现的,相对是比较重的锁,于是在 JDK 1.6 之后对 synchronized 做了一系列优化,包括偏向锁,轻量级锁,并且包括像 ConcurrentHashMap 这类并发集合都有在使用 synchronized 关键字配合 cas 来做并发保护,<p>jdk 对于 synchronized 的优化主要在于多重状态锁的升级,最初会使用偏向锁,当一个线程访问同步块并获取锁时,会在对象头和栈帧中的锁记录里存储锁偏向的线程ID,以后该线程在进入和退出同步块时不需要进行CAS操作来加锁和解锁,只需简单地测试一下对象头的Mark Word里是否存储着指向当前线程的偏向锁。引入偏向锁是为了在无多线程竞争的情况下尽量减少不必要的轻量级锁执行路径,因为轻量级锁的获取及释放依赖多次CAS原子指令,而偏向锁只需要在置换ThreadID的时候依赖一次CAS原子指令(由于一旦出现多线程竞争的情况就必须撤销偏向锁,所以偏向锁的撤销操作的性能损耗必须小于节省下来的CAS原子指令的性能消耗)。<br>而当出现线程尝试进入同步块时发现已有偏向锁,并且是其他线程时,会将锁升级成轻量级锁,并且自旋尝试获取锁,如果自旋成功则表示获取轻量级锁成功,否则将会升级成重量级锁进行阻塞,当然这里具体的还很复杂,说的比较浅薄主体还是想将原先的阻塞互斥锁进行轻量化,区分特殊情况进行加锁。</div><footer class=post-footer><div class="affiliate-creative affiliate-creative--post-end" data-affiliate-id=tencent-cloud-general><a rel="sponsored noopener" aria-label=腾讯云产品优惠 href=https://cloud.tencent.com/act/cps/redirect?redirect=2446&cps_key=2e33a1eedb380997d03618b4a5ba6ca7&from=console target=_blank><img alt=腾讯云服务器、数据库和云产品优惠 decoding=async height=419 loading=lazy src=/uploads/affiliate/tencent-cloud-general.webp width=690> </a><span class=affiliate-disclosure>推广</span></div><div class=reward-container><div>请我一杯咖啡吧!</div><button>赞赏</button><div class=post-reward><div><img alt="Nicksxs 微信" 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