jdk8對於字符串拼接的優化

首先看單個的字符串拼接：

public class Stringdeom1 {

	/**
	 * Java8會自動對  字符串加+  做優化，用到了Stringbuilder.
	 * @param args
	 */
	public static void main(String[] args) {
		String result ="";
		result += "abc";
		System.out.println(result);
	}

}

反編譯看看JVM指令:

#javap -c Stringdeom1.class
Compiled from "Stringdeom1.java"
public class Stringdeom1 {
  public Stringdeom1();
    Code:
       0: aload_0
       1: invokespecial #8                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: ldc           #16                 // String
       2: astore_1
       3: new           #18                 // class java/lang/StringBuilder
       6: dup
       7: aload_1
       8: invokestatic  #20                 // Method java/lang/String.valueOf:(Ljava/lang/Object;)Ljava/lang/String;
      11: invokespecial #26                 // Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V
      14: ldc           #29                 // String abc
      16: invokevirtual #31                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      19: invokevirtual #35                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      22: astore_1
      23: getstatic     #39                 // Field java/lang/System.out:Ljava/io/PrintStream;
      26: aload_1
      27: invokevirtual #45                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      30: return
}

可以看到在第3行生成了Stringbuilder對象，在第16行調用了append,這個很好。

 

在看看for循環多次拼接:

public class Stringdeom2 {

	/**
	 * Java8會自動對  字符串加+  做優化，用到了Stringbuilder.
	 * 但是對於循環好像有點處理的不好
	 * @param args
	 */
	public static void main(String[] args) {
		String result ="";
		for(int i=0;i<10;i++)
			result += "abc";
		System.out.println(result);
	}

}

反編譯看看:

# javap -c Stringdeom2.class
Compiled from "Stringdeom2.java"
public class Stringdeom2 {
  public Stringdeom2();
    Code:
       0: aload_0
       1: invokespecial #8                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: ldc           #16                 // String
       2: astore_1
       3: iconst_0
       4: istore_2
       5: goto          31
       8: new           #18                 // class java/lang/StringBuilder
      11: dup
      12: aload_1
      13: invokestatic  #20                 // Method java/lang/String.valueOf:(Ljava/lang/Object;)Ljava/lang/String;
      16: invokespecial #26                 // Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V
      19: ldc           #29                 // String abc
      21: invokevirtual #31                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      24: invokevirtual #35                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      27: astore_1
      28: iinc          2, 1
      31: iload_2
      32: bipush        10
      34: if_icmplt     8
      37: getstatic     #39                 // Field java/lang/System.out:Ljava/io/PrintStream;
      40: aload_1
      41: invokevirtual #45                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      44: return
}

可以看到第5行跳轉到31行，在第32行與數字10作比較,第34行決定是否跳轉到第8行繼續循環。

但是第8行才new了一個StringBuilder,這個相當於循環一次創建一個新的Stringbuilder對象，優化不算徹底。

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