Java String 面面觀

{"type":"doc","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"本文主要介紹"},{"type":"codeinline","content":[{"type":"text","text":"Java"}]},{"type":"text","text":"中與字符串相關的一些內容，主要包括"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"類的實現及其不變性、"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"相關類（"},{"type":"codeinline","content":[{"type":"text","text":"StringBuilder"}]},{"type":"text","text":"、"},{"type":"codeinline","content":[{"type":"text","text":"StringBuffer"}]},{"type":"text","text":"）的實現 以及 字符串緩存機制的用法與實現。"}]},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"String類的設計與實現"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"類的核心邏輯是通過對"},{"type":"codeinline","content":[{"type":"text","text":"char"}]},{"type":"text","text":"型數組進行封裝來實現字符串對象，但實現細節伴隨着"},{"type":"codeinline","content":[{"type":"text","text":"Java"}]},{"type":"text","text":"版本的演進也發生過幾次變化。"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"Java 6"}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public final class String implements java.io.Serializable, Comparable, CharSequence\n{\n /** The value is used for character storage. */\n private final char value[];\n /** The offset is the first index of the storage that is used. */\n private final int offset;\n /** The count is the number of characters in the String. */\n private final int count;\n /** Cache the hash code for the string */\n private int hash; // Default to 0\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在"},{"type":"codeinline","content":[{"type":"text","text":"Java 6"}]},{"type":"text","text":"中，"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"類有四個成員變量："},{"type":"codeinline","content":[{"type":"text","text":"char"}]},{"type":"text","text":"型數組"},{"type":"codeinline","content":[{"type":"text","text":"value"}]},{"type":"text","text":"、偏移量 "},{"type":"codeinline","content":[{"type":"text","text":"offset"}]},{"type":"text","text":"、字符數量 "},{"type":"codeinline","content":[{"type":"text","text":"count"}]},{"type":"text","text":"、哈希值 "},{"type":"codeinline","content":[{"type":"text","text":"hash"}]},{"type":"text","text":"。"},{"type":"codeinline","content":[{"type":"text","text":"value"}]},{"type":"text","text":"數組用來存儲字符序列， "},{"type":"codeinline","content":[{"type":"text","text":"offset"}]},{"type":"text","text":" 和 "},{"type":"codeinline","content":[{"type":"text","text":"count"}]},{"type":"text","text":" 兩個屬性用來定位字符串在"},{"type":"codeinline","content":[{"type":"text","text":"value"}]},{"type":"text","text":"數組中的位置，"},{"type":"codeinline","content":[{"type":"text","text":"hash"}]},{"type":"text","text":"屬性用來緩存字符串的"},{"type":"codeinline","content":[{"type":"text","text":"hashCode"}]},{"type":"text","text":"。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"使用"},{"type":"codeinline","content":[{"type":"text","text":"offset"}]},{"type":"text","text":"和"},{"type":"codeinline","content":[{"type":"text","text":"count"}]},{"type":"text","text":"來定位"},{"type":"codeinline","content":[{"type":"text","text":"value"}]},{"type":"text","text":"數組的目的是，可以高效、快速地共享"},{"type":"codeinline","content":[{"type":"text","text":"value"}]},{"type":"text","text":"數組，例如"},{"type":"codeinline","content":[{"type":"text","text":"substring()"}]},{"type":"text","text":"方法返回的子字符串是通過記錄"},{"type":"codeinline","content":[{"type":"text","text":"offset"}]},{"type":"text","text":"和"},{"type":"codeinline","content":[{"type":"text","text":"count"}]},{"type":"text","text":"來實現與原字符串共享"},{"type":"codeinline","content":[{"type":"text","text":"value"}]},{"type":"text","text":"數組的，而不是重新拷貝一份。"},{"type":"codeinline","content":[{"type":"text","text":"substring()"}]},{"type":"text","text":"方法實現如下："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"String(int offset, int count, char value[]) {\n\tthis.value = value; // 直接複用原數組\n\tthis.offset = offset;\n\tthis.count = count;\n}\npublic String substring(int beginIndex, int endIndex) {\n // ...... 省略一些邊界檢查的代碼 ......\n return ((beginIndex == 0) && (endIndex == count)) ? this :\n new String(offset + beginIndex, endIndex - beginIndex, value);\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"但是這種方式卻很有可能會導致內存泄漏。例如在如下代碼中："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"String bigStr = new String(new char[100000]);\nString subStr = bigStr.substring(0,2);\nbigStr = null;"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在"},{"type":"codeinline","content":[{"type":"text","text":"bigStr"}]},{"type":"text","text":"被設置爲"},{"type":"codeinline","content":[{"type":"text","text":"null"}]},{"type":"text","text":"之後，其中的"},{"type":"codeinline","content":[{"type":"text","text":"value"}]},{"type":"text","text":"數組卻仍然被"},{"type":"codeinline","content":[{"type":"text","text":"subStr"}]},{"type":"text","text":"所引用，導致垃圾回收器無法將其回收，結果雖然我們實際上僅僅需要"},{"type":"codeinline","content":[{"type":"text","text":"2"}]},{"type":"text","text":"個字符的空間，但是實際卻佔用了"},{"type":"codeinline","content":[{"type":"text","text":"100000"}]},{"type":"text","text":"個字符的空間。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在"},{"type":"codeinline","content":[{"type":"text","text":"Java 6"}]},{"type":"text","text":"中，如果想要避免這種內存泄漏情況的發生，可以使用下面的方式："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"String subStr = bigStr.substring(0,2) + \"\";\n// 或者\nString subStr = new String(bigStr.substring(0,2));"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在語句執行完之後，"},{"type":"codeinline","content":[{"type":"text","text":"substring"}]},{"type":"text","text":"方法返回的匿名"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"對象由於沒有被別的對象引用，所以能夠被垃圾回收器回收，不會繼續引用"},{"type":"codeinline","content":[{"type":"text","text":"bigStr"}]},{"type":"text","text":"中的"},{"type":"codeinline","content":[{"type":"text","text":"value"}]},{"type":"text","text":"數組，從而避免了內存泄漏。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"Java 7 & Java 8"}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public final class String implements java.io.Serializable, Comparable, CharSequence {\n /** The value is used for character storage. */\n private final char value[];\n /** Cache the hash code for the string */\n private int hash; // Default to 0\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在"},{"type":"codeinline","content":[{"type":"text","text":"Java 7"}]},{"type":"text","text":"-"},{"type":"codeinline","content":[{"type":"text","text":"Java 8"}]},{"type":"text","text":"中，"},{"type":"codeinline","content":[{"type":"text","text":"Java"}]},{"type":"text","text":" 對 "},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":" 類做了一些改變。"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":" 類中不再有 "},{"type":"codeinline","content":[{"type":"text","text":"offset"}]},{"type":"text","text":" 和 "},{"type":"codeinline","content":[{"type":"text","text":"count"}]},{"type":"text","text":" 兩個成員變量了。"},{"type":"codeinline","content":[{"type":"text","text":"substring()"}]},{"type":"text","text":"方法也不再共享 "},{"type":"codeinline","content":[{"type":"text","text":"value"}]},{"type":"text","text":"數組，而是從指定位置重新拷貝一份"},{"type":"codeinline","content":[{"type":"text","text":"value"}]},{"type":"text","text":"數組，從而解決了使用該方法可能導致的內存泄漏問題。"},{"type":"codeinline","content":[{"type":"text","text":"substring()"}]},{"type":"text","text":"方法實現如下："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public String(char value[], int offset, int count) {\n // ...... 省略一些邊界檢查的代碼 ......\n\n // 從原數組拷貝\n this.value = Arrays.copyOfRange(value, offset, offset+count); \n}\npublic String substring(int beginIndex, int endIndex) {\n // ...... 省略一些邊界檢查的代碼 ......\n return ((beginIndex == 0) && (endIndex == value.length)) ? this\n : new String(value, beginIndex, subLen);\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"Java 9"}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public final class String implements java.io.Serializable, Comparable, CharSequence {\n /** The value is used for character storage. */\n private final byte[] value;\n /** The identifier of the encoding used to encode the bytes in {@code value}. */\n private final byte coder;\n /** Cache the hash code for the string */\n private int hash; // Default to 0\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"爲了節省內存空間，"},{"type":"codeinline","content":[{"type":"text","text":"Java 9"}]},{"type":"text","text":"中對"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"的實現方式做了優化，"},{"type":"codeinline","content":[{"type":"text","text":"value"}]},{"type":"text","text":"成員變量從"},{"type":"codeinline","content":[{"type":"text","text":"char[]"}]},{"type":"text","text":"類型改爲了"},{"type":"codeinline","content":[{"type":"text","text":"byte[]"}]},{"type":"text","text":"類型，同時新增了一個"},{"type":"codeinline","content":[{"type":"text","text":"coder"}]},{"type":"text","text":"成員變量。我們知道"},{"type":"codeinline","content":[{"type":"text","text":"Java"}]},{"type":"text","text":"中"},{"type":"codeinline","content":[{"type":"text","text":"char"}]},{"type":"text","text":"類型佔用的是兩個字節，對於只佔用一個字節的字符（例如，"},{"type":"codeinline","content":[{"type":"text","text":"a-z"}]},{"type":"text","text":"，"},{"type":"codeinline","content":[{"type":"text","text":"A-Z"}]},{"type":"text","text":"）就顯得有點浪費，所以"},{"type":"codeinline","content":[{"type":"text","text":"Java 9"}]},{"type":"text","text":"中將"},{"type":"codeinline","content":[{"type":"text","text":"char[]"}]},{"type":"text","text":"改爲"},{"type":"codeinline","content":[{"type":"text","text":"byte[]"}]},{"type":"text","text":"來存儲字符序列，而新屬性 "},{"type":"codeinline","content":[{"type":"text","text":"coder"}]},{"type":"text","text":" 的作用就是用來表示"},{"type":"codeinline","content":[{"type":"text","text":"value"}]},{"type":"text","text":"數組中存儲的是雙字節編碼的字符還是單字節編碼的字符。"},{"type":"codeinline","content":[{"type":"text","text":"coder"}]},{"type":"text","text":" 屬性可以有 "},{"type":"codeinline","content":[{"type":"text","text":"0"}]},{"type":"text","text":" 和 "},{"type":"codeinline","content":[{"type":"text","text":"1"}]},{"type":"text","text":" 兩個值，"},{"type":"codeinline","content":[{"type":"text","text":"0"}]},{"type":"text","text":" 代表 "},{"type":"codeinline","content":[{"type":"text","text":"Latin-1"}]},{"type":"text","text":"（單字節編碼），"},{"type":"codeinline","content":[{"type":"text","text":"1"}]},{"type":"text","text":" 代表 "},{"type":"codeinline","content":[{"type":"text","text":"UTF-16"}]},{"type":"text","text":"（雙字節編碼）。在創建字符串的時候如果判斷所有字符都可以用單字節來編碼，則使用"},{"type":"codeinline","content":[{"type":"text","text":"Latin-1"}]},{"type":"text","text":"來編碼以壓縮空間，否則使用"},{"type":"codeinline","content":[{"type":"text","text":"UTF-16"}]},{"type":"text","text":"編碼。主要的構造函數實現如下："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"String(char[] value, int off, int len, Void sig) {\n if (len == 0) {\n this.value = \"\".value;\n this.coder = \"\".coder;\n return;\n }\n if (COMPACT_STRINGS) {\n byte[] val = StringUTF16.compress(value, off, len); // 嘗試壓縮字符串，使用單字節編碼存儲\n if (val != null) { // 壓縮成功，可以使用單字節編碼存儲\n this.value = val;\n this.coder = LATIN1;\n return;\n }\n }\n // 否則，使用雙字節編碼存儲\n this.coder = UTF16;\n this.value = StringUTF16.toBytes(value, off, len);\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"String類的不變性"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們注意到"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"類是用"},{"type":"codeinline","content":[{"type":"text","text":"final"}]},{"type":"text","text":"修飾的；所有的屬性都是聲明爲"},{"type":"codeinline","content":[{"type":"text","text":"private"}]},{"type":"text","text":"的；並且除了"},{"type":"codeinline","content":[{"type":"text","text":"hash"}]},{"type":"text","text":"屬性之外的其他屬性也都是用"},{"type":"codeinline","content":[{"type":"text","text":"final"}]},{"type":"text","text":"修飾。這保證了："}]},{"type":"numberedlist","attrs":{"start":"1","normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"類由"},{"type":"codeinline","content":[{"type":"text","text":"final"}]},{"type":"text","text":"修飾，所以無法通過繼承"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"類改變其語義；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"所有的屬性都是聲明爲"},{"type":"codeinline","content":[{"type":"text","text":"private"}]},{"type":"text","text":"的， 所以無法在"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"外部"},{"type":"text","marks":[{"type":"strong"}],"text":"直接"},{"type":"text","text":"訪問或修改其屬性；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"除了"},{"type":"codeinline","content":[{"type":"text","text":"hash"}]},{"type":"text","text":"屬性之外的其他屬性都是用"},{"type":"codeinline","content":[{"type":"text","text":"final"}]},{"type":"text","text":"修飾，表示這些屬性在初始化賦值後不可以再修改。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"上述的定義共同實現了"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"類一個重要的特性 —— **不變性**，即 "},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":" 對象一旦創建成功，就不能再對它進行任何修改。"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"提供的方法"},{"type":"codeinline","content":[{"type":"text","text":"substring()"}]},{"type":"text","text":"、"},{"type":"codeinline","content":[{"type":"text","text":"concat()"}]},{"type":"text","text":"、"},{"type":"codeinline","content":[{"type":"text","text":"replace()"}]},{"type":"text","text":"等方法返回值都是新創建的"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"對象，而不是原來的"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"對象。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"hash"}]},{"type":"text","text":"屬性不是"},{"type":"codeinline","content":[{"type":"text","text":"final"}]},{"type":"text","text":"的原因是："},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"的"},{"type":"codeinline","content":[{"type":"text","text":"hashCode"}]},{"type":"text","text":"並不需要在創建字符串時立即計算並賦值，而是在"},{"type":"codeinline","content":[{"type":"text","text":"hashCode()"}]},{"type":"text","text":"方法被調用時才需要進行計算。"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"爲什麼String類要設計爲不可變的？"}]},{"type":"numberedlist","attrs":{"start":"1","normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"保證 "},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":" 對象的安全性。"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"被廣泛用作"},{"type":"codeinline","content":[{"type":"text","text":"JDK"}]},{"type":"text","text":"中作爲參數、返回值，例如網絡連接，打開文件，類加載，等等。如果 "},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":" 對象是可變的，那麼 "},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":" 對象將可能被惡意修改，引發安全問題。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"線程安全。"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"類的不可變性天然地保證了其線程安全的特性。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"保證了"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"對象的"},{"type":"codeinline","content":[{"type":"text","text":"hashCode"}]},{"type":"text","text":"的不變性。"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"類的不可變性，保證了其"},{"type":"codeinline","content":[{"type":"text","text":"hashCode"}]},{"type":"text","text":"值能夠在第一次計算後進行緩存，之後無需重複計算。這使得"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"對象很適合用作"},{"type":"codeinline","content":[{"type":"text","text":"HashMap"}]},{"type":"text","text":"等容器的"},{"type":"codeinline","content":[{"type":"text","text":"Key"}]},{"type":"text","text":"，並且相比其他對象效率更高。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":4,"align":null,"origin":null},"content":[{"type":"text","text":"實現"},{"type":"codeinline","content":[{"type":"text","text":"字符串常量池"}]},{"type":"text","text":"。"},{"type":"codeinline","content":[{"type":"text","text":"Java"}]},{"type":"text","text":"爲字符串對象設計了"},{"type":"codeinline","content":[{"type":"text","text":"字符串常量池"}]},{"type":"text","text":"來共享字符串，節省內存空間。如果字符串是可變的，那麼字符串對象便無法共享。因爲如果改變了其中一個對象的值，那麼其他對象的值也會相應發生變化。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"與String類相關的類"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"除了"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"類之外，還有兩個與"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"類相關的的類："},{"type":"codeinline","content":[{"type":"text","text":"StringBuffer"}]},{"type":"text","text":"和"},{"type":"codeinline","content":[{"type":"text","text":"StringBuilder"}]},{"type":"text","text":"，這兩個類可以看作是"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"類的可變版本，提供了對字符串修改的各種方法。兩者的區別在於"},{"type":"codeinline","content":[{"type":"text","text":"StringBuffer"}]},{"type":"text","text":"是線程安全的而"},{"type":"codeinline","content":[{"type":"text","text":"StringBuilder"}]},{"type":"text","text":"不是線程安全的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"StringBuffer 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，這個常量池中主要存儲兩大類常量："}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"代碼中的"},{"type":"codeinline","content":[{"type":"text","text":"字面量"}]},{"type":"text","text":"或者"},{"type":"codeinline","content":[{"type":"text","text":"常量表達式"}]},{"type":"text","text":"的值；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"符號引用，包括：類和接口的全限定名、字段的名稱和描述符、方法的名稱和描述符等。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"2. 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String Literals"}]},{"type":"text","text":"）。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"字符串常量表達式"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"另外一種會被隱式緩存的字符串是 "},{"type":"text","marks":[{"type":"strong"}],"text":"字符串常量表達式"},{"type":"text","text":"。"},{"type":"codeinline","content":[{"type":"text","text":"常量表達式"}]},{"type":"text","text":"指的是表示簡單類型值或"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"對象的表達式，可以簡單理解爲"},{"type":"codeinline","content":[{"type":"text","text":"常量表達式"}]},{"type":"text","text":"就是在編譯期間就能確定值的表達式。"},{"type":"codeinline","content":[{"type":"text","text":"字符串常量表達式"}]},{"type":"text","text":"就是表示"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"對象的常量表達式。例如："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"int a = 1 + 2;\ndouble d = 10 + 2.01;\nboolean b = true & false;\nString str1 = \"abc\" + 123;\n\nfinal int num = 456;\nString str2 = \"abc\" +456;"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"Java"}]},{"type":"text","text":"會在執行過程中爲"},{"type":"codeinline","content":[{"type":"text","text":"字符串常量表達式"}]},{"type":"text","text":"創建"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"對象並加入"},{"type":"codeinline","content":[{"type":"text","text":"字符串常量池"}]},{"type":"text","text":"中。例如，上面的代碼中，會分別創建"},{"type":"codeinline","content":[{"type":"text","text":"\"abc123\""}]},{"type":"text","text":"和"},{"type":"codeinline","content":[{"type":"text","text":"\"abc456\""}]},{"type":"text","text":"兩個"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"對象，這兩個"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"對象會被緩存到"},{"type":"codeinline","content":[{"type":"text","text":"字符串常量池"}]},{"type":"text","text":"中，"},{"type":"codeinline","content":[{"type":"text","text":"str1"}]},{"type":"text","text":"會指向常量池中值爲"},{"type":"codeinline","content":[{"type":"text","text":"\"abc123\""}]},{"type":"text","text":"的"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"對象，"},{"type":"codeinline","content":[{"type":"text","text":"str2"}]},{"type":"text","text":"會指向常量池中值爲"},{"type":"codeinline","content":[{"type":"text","text":"\"abc456\""}]},{"type":"text","text":"的"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"對象。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"關於"},{"type":"codeinline","content":[{"type":"text","text":"常量表達式"}]},{"type":"text","text":"更加詳細的內容請參閱"},{"type":"codeinline","content":[{"type":"text","text":"Java語言規範"}]},{"type":"text","text":"（"},{"type":"link","attrs":{"href":"https://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.28","title":""},"content":[{"type":"text","text":"JLS - 15.28 Constant Expressions"}]},{"type":"text","text":"）。"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"2. 主動緩存 - String.intern()方法"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"除了聲明爲"},{"type":"codeinline","content":[{"type":"text","text":"字符串字面量"}]},{"type":"text","text":"/"},{"type":"codeinline","content":[{"type":"text","text":"字符串常量表達式"}]},{"type":"text","text":"之外，通過其他方式得到的"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"對象也可以主動加入"},{"type":"codeinline","content":[{"type":"text","text":"字符串常量池"}]},{"type":"text","text":"中。例如："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"String str = new String(\"123\") + new String(\"456\");\nstr.intern();"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在上面的代碼中，在執行完第一句後，常量池中存在內容爲"},{"type":"codeinline","content":[{"type":"text","text":"\"123\""}]},{"type":"text","text":"和"},{"type":"codeinline","content":[{"type":"text","text":"\"456\""}]},{"type":"text","text":"的兩個"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"對象，但是不存在"},{"type":"codeinline","content":[{"type":"text","text":"\"123456\""}]},{"type":"text","text":"的"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"對象，但在執行完"},{"type":"codeinline","content":[{"type":"text","text":"str.intern();"}]},{"type":"text","text":"之後，內容爲"},{"type":"codeinline","content":[{"type":"text","text":"\"123456\""}]},{"type":"text","text":"的"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"對象也加入到了"},{"type":"codeinline","content":[{"type":"text","text":"字符串常量池"}]},{"type":"text","text":"中。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們通過"},{"type":"codeinline","content":[{"type":"text","text":"String.intern()"}]},{"type":"text","text":"方法的註釋來看下其具體的緩存機制："}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"When the intern method is invoked, if the pool already contains a string equal to this String object as determined by the equals(Object) method, then the string from the pool is returned. Otherwise, this String object is added to the pool and a reference to this String object is returned."}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"It follows that for any two strings s and t, s.intern() == t.intern() is true if and only if s.equals(t) is true."}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"簡單翻譯一下："}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"當調用 "},{"type":"codeinline","content":[{"type":"text","text":"intern"}]},{"type":"text","text":" 方法時，如果常量池中已經包含相同內容的字符串（字符串內容相同由 "},{"type":"codeinline","content":[{"type":"text","text":"equals (Object)"}]},{"type":"text","text":" 方法確定，對於 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t.intern()"}]},{"type":"text","text":"的結果爲"},{"type":"codeinline","content":[{"type":"text","text":"true"}]},{"type":"text","text":"。"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"String對象被緩存在哪裏，如何組織起來的？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"HotSpot VM"}]},{"type":"text","text":"中，有一個用來記錄緩存的"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"對象的全局表，叫做"},{"type":"codeinline","content":[{"type":"text","text":"StringTable"}]},{"type":"text","text":"，結構及實現方式都類似於"},{"type":"codeinline","content":[{"type":"text","text":"Java"}]},{"type":"text","text":"中的"},{"type":"codeinline","content":[{"type":"text","text":"HashMap"}]},{"type":"text","text":"或者"},{"type":"codeinline","content":[{"type":"text","text":"HashSet"}]},{"type":"text","text":"，是一個使用拉鍊法解決哈希衝突的哈希表，可以簡單理解爲"},{"type":"codeinline","content":[{"type":"text","text":"HashSet"}]},{"type":"text","text":"，注意它只存儲對"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"對象的引用，而不存儲"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"對象實例。 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class Main {\n public static void main(String[] args) {\n String str1 = \"123\" + 123; // 字符串常量表達式\n String str2 = \"123456\"; // 字面量\n String str3 = \"123\" + 456; //字符串常量表達式\n }\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"字節碼分析"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們對上述代碼編譯之後使用"},{"type":"codeinline","content":[{"type":"text","text":"javap"}]},{"type":"text","text":"來觀察一下字節碼文件，爲了節省篇幅，只摘取了相關的部分：常量池表部分以及"},{"type":"codeinline","content":[{"type":"text","text":"main"}]},{"type":"text","text":"方法信息部分："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"Constant pool:\n #1 = Methodref #5.#23 // java/lang/Object.\"\":()V\n #2 = String #24 // 123123\n #3 = String #25 // 123456\n // ...... 省略 ......\n #24 = Utf8 123123\n #25 = Utf8 123456\n \n // ...... 省略 ......\n\n public static void main(java.lang.String[]);\n descriptor: ([Ljava/lang/String;)V\n flags: ACC_PUBLIC, ACC_STATIC\n Code:\n stack=1, locals=4, args_size=1\n 0: ldc #2 // String 123123\n 2: astore_1\n 3: ldc #3 // String 123456\n 5: astore_2\n 6: ldc #3 // String 123456\n 8: astore_3\n 9: return\n LineNumberTable:\n line 7: 0\n line 8: 3\n line 9: 6\n line 10: 9\n LocalVariableTable:\n Start Length Slot Name Signature\n 0 10 0 args [Ljava/lang/String;\n 3 7 1 str1 Ljava/lang/String;\n 6 4 2 str2 Ljava/lang/String;\n 9 1 3 str3 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\"123\" + b + \"567\";\nString s3 = \"1234567\";\nSystem.out.println(s1 == s2);\nSystem.out.println(s1 == s3);\nSystem.out.println(s2 == s3);"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"結果："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"false\ntrue\nfalse"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"解釋："}]},{"type":"numberedlist","attrs":{"start":"1","normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"第三行，因爲"},{"type":"codeinline","content":[{"type":"text","text":"a"}]},{"type":"text","text":"被定義爲常量，所以"},{"type":"codeinline","content":[{"type":"text","text":"\"123\" + a + 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s1 = new String(\"123\");\nString s2 = s1.intern();\nString s3 = \"123\";\nSystem.out.println(s1 == s2); \nSystem.out.println(s1 == s3); \nSystem.out.println(s2 == s3);"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"結果："}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"false\nfalse\ntrue"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"解釋："}]},{"type":"numberedlist","attrs":{"start":"1","normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"第一行，"},{"type":"codeinline","content":[{"type":"text","text":"\"123\""}]},{"type":"text","text":"是一個"},{"type":"codeinline","content":[{"type":"text","text":"字符串字面量"}]},{"type":"text","text":"，所以首先在"},{"type":"codeinline","content":[{"type":"text","text":"字符串常量池"}]},{"type":"text","text":"中創建了一個"},{"type":"codeinline","content":[{"type":"text","text":"\"123\""}]},{"type":"text","text":"對象，然後使用"},{"type":"codeinline","content":[{"type":"text","text":"String"}]},{"type":"text","text":"的構造函數在堆中創建了一個"},{"type":"codeinline","content":[{"type":"text","text":"\"123\""}]},{"type":"text","text":"對象，"},{"type":"codeinline","content":[{"type":"text","text":"s1"}]},{"type":"text","text":"指向堆中的"},{"type":"codeinline","content":[{"type":"text","text":"\"123\""}]},{"type":"text","text":"；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"第二行，因爲"},{"type":"codeinline","content":[{"type":"text","text":"字符串常量池"}]},{"type":"text","text":"中已經有了"},{"type":"codeinline","content":[{"type":"text","text":"\"123\""}]},{"type":"text","text":"，所以"},{"type":"codeinline","content":[{"type":"text","text":"s2"}]},{"type":"text","text":"指向"},{"type":"codeinline","content":[{"type":"text","text":"字符串常量池"}]},{"type":"text","text":"中的"},{"type":"codeinline","content":[{"type":"text","text":"\"123\""}]},{"type":"text","text":"；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"第三行，同樣因爲"},{"type":"codeinline","content":[{"type":"text","text":"字符串常量池"}]},{"type":"text","text":"中已經有了"},{"type":"codeinline","content":[{"type":"text","text":"\"123\""}]},{"type":"text","text":"，所以"},{"type":"codeinline","content":[{"type":"text","text":"s3"}]},{"type":"text","text":"指向"},{"type":"codeinline","content":[{"type":"text","text":"字符串常量池"}]},{"type":"text","text":"中的"},{"type":"codeinline","content":[{"type":"text","text":"\"123\""}]},{"type":"text","text":"。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"horizontalrule"},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"String s1 = String.valueOf(\"123\");\nString s2 = s1.intern();\nString s3 = \"123\";\nSystem.out.println(s1 == s2); \nSystem.out.println(s1 == s3); \nSystem.out.println(s2 == s3); 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