1.橋接方法簡介
橋接方法是jdk1.5引入泛型后，為使java泛型方法生成的位元組碼與jdk1.5版本之前的位元組碼兼容由編譯器自動生成的，
可用method.isBridge() 判斷method是否是橋接方法，在生成的位元組碼中會有flags標記 ACC_BRIDGE, ACC_SYNTHETIC ，根據來自深入理解java虛擬機的一張訪問標志圖可以看到 ACC_BRIDGE表示方法是由編譯器產生的橋接方法，ACC_SYNTHETIC表示方法由編譯器自動產生不屬于原始碼，
2. 什么時候會生成橋接方法
當子類繼承父類（繼承介面）實作抽象泛型方法的時候，編譯器會為子類自動生成橋接方法
#父類
public abstract class SuperClass<T> {
public abstract T get(T t) ;
}
#子類
public class SubClass extends SuperClass<String> {
@Override
public String get(String s) {
return s;
}
}使用javap -v SubClass.class命令查看類SubClass的位元組碼：
Classfile /Users/xudong/project-maven/test/person-study/dubbo-provider/target/classes/com/monian/dubbo/provider/study/generic/SubClass.class
Last modified 2022年7月25日; size 777 bytes
MD5 checksum 1328a7043cde4b809a156e7a239335a6
Compiled from "SubClass.java"
public class com.monian.dubbo.provider.study.generic.SubClass extends com.monian.dubbo.provider.study.generic.SuperClass<java.lang.String>
minor version: 0
major version: 52
flags: (0x0021) ACC_PUBLIC, ACC_SUPER
this_class: #4 // com/monian/dubbo/provider/study/generic/SubClass
super_class: #5 // com/monian/dubbo/provider/study/generic/SuperClass
interfaces: 0, fields: 0, methods: 3, attributes: 2
Constant pool:
#1 = Methodref #5.#23 // com/monian/dubbo/provider/study/generic/SuperClass."<init>":()V
#2 = Class #24 // java/lang/String
#3 = Methodref #4.#25 // com/monian/dubbo/provider/study/generic/SubClass.get:(Ljava/lang/String;)Ljava/lang/String;
#4 = Class #26 // com/monian/dubbo/provider/study/generic/SubClass
#5 = Class #27 // com/monian/dubbo/provider/study/generic/SuperClass
#6 = Utf8 <init>
#7 = Utf8 ()V
#8 = Utf8 Code
#9 = Utf8 LineNumberTable
#10 = Utf8 LocalVariableTable
#11 = Utf8 this
#12 = Utf8 Lcom/monian/dubbo/provider/study/generic/SubClass;
#13 = Utf8 get
#14 = Utf8 (Ljava/lang/String;)Ljava/lang/String;
#15 = Utf8 s
#16 = Utf8 Ljava/lang/String;
#17 = Utf8 MethodParameters
#18 = Utf8 (Ljava/lang/Object;)Ljava/lang/Object;
#19 = Utf8 Signature
#20 = Utf8 Lcom/monian/dubbo/provider/study/generic/SuperClass<Ljava/lang/String;>;
#21 = Utf8 SourceFile
#22 = Utf8 SubClass.java
#23 = NameAndType #6:#7 // "<init>":()V
#24 = Utf8 java/lang/String
#25 = NameAndType #13:#14 // get:(Ljava/lang/String;)Ljava/lang/String;
#26 = Utf8 com/monian/dubbo/provider/study/generic/SubClass
#27 = Utf8 com/monian/dubbo/provider/study/generic/SuperClass
{
public com.monian.dubbo.provider.study.generic.SubClass();
descriptor: ()V
flags: (0x0001) ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method com/monian/dubbo/provider/study/generic/SuperClass."<init>":()V
4: return
LineNumberTable:
line 7: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/monian/dubbo/provider/study/generic/SubClass;
public java.lang.String get(java.lang.String);
descriptor: (Ljava/lang/String;)Ljava/lang/String;
flags: (0x0001) ACC_PUBLIC
Code:
stack=1, locals=2, args_size=2
0: aload_1
1: areturn
LineNumberTable:
line 11: 0
LocalVariableTable:
Start Length Slot Name Signature
0 2 0 this Lcom/monian/dubbo/provider/study/generic/SubClass;
0 2 1 s Ljava/lang/String;
MethodParameters:
Name Flags
s
public java.lang.Object get(java.lang.Object);
descriptor: (Ljava/lang/Object;)Ljava/lang/Object;
flags: (0x1041) ACC_PUBLIC, ACC_BRIDGE, ACC_SYNTHETIC
Code:
stack=2, locals=2, args_size=2
0: aload_0
1: aload_1
2: checkcast #2 // class java/lang/String
5: invokevirtual #3 // Method get:(Ljava/lang/String;)Ljava/lang/String;
8: areturn
LineNumberTable:
line 7: 0
LocalVariableTable:
Start Length Slot Name Signature
0 9 0 this Lcom/monian/dubbo/provider/study/generic/SubClass;
MethodParameters:
Name Flags
s synthetic
}
Signature: #20 // Lcom/monian/dubbo/provider/study/generic/SuperClass<Ljava/lang/String;>;
SourceFile: "SubClass.java"可以看到位元組碼中有兩個get方法，第二個方法引數和回傳值型別都是java.lang.Object 并且可以看到flags有相應標志ACC_BRIDGE, ACC_SYNTHETIC說明此方法就是有編譯器自動生成的橋接方法，再看code屬性：
aload_0：把this變數裝載到運算元堆疊中
aload_1：把方法變數s裝載到運算元堆疊中
checkcast # 2：校驗堆疊頂變數s是否為java.lang.String型別
invokevirtual # 3： 呼叫方法 public String get(String s)
areturn： 回傳結果
根據上述code解釋可以看出編譯器生成的橋接方法為這個樣子的，橋接方法實際上呼叫了實際的泛型方法
public String get(String s) {
return s;
}
#橋接方法
public Object get(Object s) {
return get((String) s);
}
泛型-型別擦除
public class SubClass extends SuperClass<String> {
@Override
public String get(String s) {
return s;
}
public static void main(String[] args) {
SuperClass subClass = new SubClass();
Object s = "hello world";
System.out.println(subClass.get(s));
}
}java的泛型在運行時會進行泛型擦除替換成非泛型上邊界，java虛擬機無法知道準確的型別， 上述代碼能編譯通過并且會呼叫子類SubClass的橋接方法由橋接方法再去呼叫實際泛型方法，如果定義為SuperClass<String> subClass = new SubClass(); 那么get方法入參只能為String變數，因為編譯器在編譯期間會進行型別校驗，不符合型別將直接報編譯失敗，
3. 為什么生成泛型方法
{
public com.monian.dubbo.provider.study.generic.SuperClass();
descriptor: ()V
flags: (0x0001) ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
LineNumberTable:
line 7: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/monian/dubbo/provider/study/generic/SuperClass;
LocalVariableTypeTable:
Start Length Slot Name Signature
0 5 0 this Lcom/monian/dubbo/provider/study/generic/SuperClass<TT;>;
public abstract T get(T);
descriptor: (Ljava/lang/Object;)Ljava/lang/Object;
flags: (0x0401) ACC_PUBLIC, ACC_ABSTRACT
MethodParameters:
Name Flags
t
Signature: #18 // (TT;)TT;
}為了能夠正確的編譯，可以看到原始碼中父類SuperClass get方法引數型別為T（T t），而在位元組碼層面可以看到，經過編譯后，get方法入參和回傳值型別都為Object，
可以想象一下，如果沒有編譯器自動生成的橋接方法，那么編譯是不會通過的，父類SubClass get方法經過編譯后入參和回傳值型別都為Object，而子類get方法入參和回傳值型別為String，子類并沒有重寫父類的get方法（重寫：訪問的方法的實作程序進行重新撰寫, 回傳值和形參都不能改變），所有編譯器需要生成一個橋接方法，Object get(Object) 就可以編譯通過了，
4. 根據橋接方法獲取實際泛型方法
主要借助Spring的BridgeMethodResolver#findBridgedMethod找到被橋接的方法，原理是首先找到類宣告的所有方法，找到與橋接方法簡單名稱和方法引數數量相同的候選方法，若只要一個則直接回傳，若有多個則回圈判斷方法引數型別是否相同或者候選方法都有相同的方法簽名則從其中任選一個方法作為被橋接的方法，
@Slf4j
public class SubClass extends SuperClass<String> {
@Override
public String get(String s) {
return s;
}
public static void main(String[] args) throws Exception {
SubClass subClass = new SubClass();
Method bridgeMethod = subClass.getClass().getDeclaredMethod("get", Object.class);
log.info("bridgeMethod is bridge:" + bridgeMethod.isBridge());
log.info("bridgeMethod:" + bridgeMethod.toString());
// 實際泛型方法
Method actualMethod = subClass.getClass().getDeclaredMethod("get", String.class);
log.info("actualMethod:" + actualMethod.toString());
// 通過spring #BridgeMethodResolver由橋接方法獲取到實際泛型方法
Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(bridgeMethod);
log.info("bridgedMethod:" + bridgedMethod.toString());
}
}輸出如下：
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標籤：Java
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